At the request of Professor M. Amakiir, on the subject of Nethermancy I, L.I. Celess Soulbinder, present:
A Demonological Study of the Beings of the Nether:
In which I shall discuss those demons that a Nethermancer shall learn dominion of, and all other known inhabitants of the void.
The imp is at the bottom of demonic society and, fittingly, the first minion a Warlock will learn dominion over. To the Warlock, an imp is a useful companion, both able to attack foes from a distance by throwing fireballs and able to protect its master using magical fire wards. The imp also has an incredibly useful ability known as The Blood Pact in which the imp and its master share a link with each other, infusing the masters blood with fel energies, boosting the blood flow around the body and granting the master demonic stamina. 
In demonic society, the imp is used as a servant, a petty underling to more powerful demonic entities, running errands or serving as a meat shield should the imps infernal master come under attack.
The satyr of Maraudon use the imps in an unusual way. By reversing their Blood Pact ability, the Corruptors (as they are known) serve to weaken the satyrs foes by causing a form of necrosis within the haemoglobin of the targets blood, stymieing the blood flow and substantially weakening the victim. 
Imps have been utilised for many situations by the Legion, from simple servants performing menial tasks to spies. The Imp is essentially the subversive grunt of the Legion's forces and are often granted as gifts to those who would do the Legion's bidding.
Imps have been seen in use for other means across Azeroth. The imp Jeevee is used by many Warlocks in search of gaining a Xorothian Dreadsteed, as the imp is trained in the creation of the parchment required for the summoning and in setting up the components for the ritual. Jeevee is a testament for the versatility and, indeed, utility of imps. I myself utilise Abamat, my own personal imp servant frequently and most of my correspondence - including this essay - is dictated directly to him.
Similar in appearance to many of the elementals, it is quite usual for them to be referred to, incorrectly, as shadow elementals. This name, though understandable due to their nature as beings consisting of purest shadow and misery, is voided by the creatures demonic nature - they are not elementals.
The Voidwalker is a creature that feels no pain and is ocassionally used in the Burning Legion as meat-shields in the first wave of an attack because of this, attacking voraciously with large clawed hands, though Voidwalkers only generally are introduced a fair stretch into the invasion after the main force. Voidwalkers vary in size, from the relatively small ones that Warlocks bind to themselves, to the far larger, armoured demons found wandering the Void Fields, and Void Ridge of Hellfire Peninsula. In fact, it has since come to understanding that the Voidwalkers in the Void Fields and those found within the Arcatraz of Tempest Keep are not under the juresdiction of the Legion, but more a mercenary band also hailing from the Twisting Nether.
Because of the Voidwalkers inability to feel pain, they are greatly valued by those Warlocks who fear pain, often used to tank a foe and to steal the foes attention away from the Warlock, and during moments of rest, the creature can feed on shadows and regain its strength. For this purpose, the Voidwalker is one of the most commonly used minions in a Warlock's arsenal.
Voidwalkers do not follow the religion of the Burning Legion, instead serving the Legion merely to further their own goals of spreading misery and destroying the material, leaving nothing but the shadowy nothingness upon which they thrive.
For a long time, Zangkath, my own Voidwalker minion, served incredibly well on my travels, protecting me from foes much larger than myself, long enough for my curses and spells to utterly vanquish the foe.
A Succubus (plural, Succubae or Succubi) is a demon of seduction, appearing in the form of a beautiful woman, though with darker, sinister features such as horns and bat-like wings. A mistress of lust, she is incredibly well taught in the arts of seduction and has a deep understanding of the power of love and lust and able to hold back almost any humanoid with her abilities.
A smart Warlock will learn how to use a Succubus abilities in crowd control situations, to seduce certain foes and hold them helpless, charmed. A Succubus also makes a good travelling companion as one of only two of a Warlocks minions to be notable for their social skills (along with imps).
Within demonic society, the Succubus fulfils one of many roles. Often a succubus is sent as a spy or infiltrator into a rival clans hold , or into the mortal realms as advisors to more nefarious individuals within the world, twisting their words and imprinting seeds of corruption from their true masters; and due to their charming nature, the Succubus will often go undetected. 
To this end, many of the more subtle Warlocks value the abilities of the Succubus to influence their rivals and to further their own ends.
On the road, my own Succubus, Zahna, has proved incredibly useful. Often when I have been attacked by multiple foes, she has seduced one long enough to steal his attention from me, and she is quite talkative. It should be stressed at this point, that part of the Contract of Binding denotes that the Master is immune to the Succubus powers, and the Contract protects the Master indefinitely.
Also Felhunter, Felhound...
The Felhunters are horned dog-like demons, bristling with black spines and with two large tendrils protruding from their neck. These tendrils serve two purposes, firstly as the demons sensors, as the demon has no eyes, nose nor ears, instead using these tendrils to sense their victims spiritual resonance, effectively feeling where their prey is before giving chase. The demon then attacks by latching these tendrils to the victim and draining them of any magical essence . Thus, naturally, the Fel Beasts have an affinity for spell-casters and those who utilise magical energies, but are still as fond of the flesh of even the most mundane creature. Once a Fel Beast has absorbed enough magical energy, it is able to divide into multiple others of its kind.
Fel Beastss in the Burning Legion are used by the Pit Lords much like attack dogs, chasing down deserters on both sides, and collectively pulling down foes many times their size. Due to their penchant for magicians, they are equally as often employed as Mage Hunters, seeking out spell-casters and silencing them using its anti-magical aura .
A Fel Beasts blood is also laced with demonic poisons, and every drop that is leased from the beast weakens its attacker substantially  and a pack of Fel Beasts can pull down a foe substantially larger than them, as every wounded Felhunter, weakens their prey and the collective mass of snarling maws can pull down the atrophied target.
Under a Warlock's dominion, a Fel Beast is usually used exactly to these roles, as an attack dog to pull down more agile foes, or as an effective anti-mage. Also, due to their sharpened, non-optical based senses, a Fel Beast can effectively detect foes utilising stealth techniques.
The first Fel Beasts were brought onto Azeroth by the Houndmaster, Hakkar of the Legion who was attuned to their energies and utilised them as fore-runners and spies whilst the Highborn continued summoning the rest of the Legion.
Drooghun, my own Felhunter, has served well on numerous times, both when hunting the magicians of the world and in alerting me to assassins. Without such sensing, I do fear I would not be dictating this essay presently.
Of the Fel Guard:
Large humanoid demons, the Fel Guard stand towering above elves and match even the height of the Tauren at approximately 9 foot. Behind their neck their form mutates to three large spines, and most Felguards are equipped with large weapons (most commonly axes or mauls, and ocassionaly shields) and bear incredibly durable Demon-Bronze armour, making them formidable opponents indeed.
A Fel Guard is an engine of baleful fury and wrath, capable of flooring an opponent in a single charge before bringing a swift end to their victim with a crushing blow to the head or a severing cleave through the neck. They are attentive and muscular, combining brute force and agility to deadly effect.
Within the armies of the Legion, the Fel Guard is a basic foot soldier, usually under the direct command of a Doomguard who in turn serves the Pit Lords. Members of the Mo'Arg demons, Fel Guard are trained only to fight, though rarely the more intelligent have been seen to lead small task-forces, or more commonly as a keeper for a Felhound.
Within places like Jaednar, Fel Guard are used as Elite warriors on their own, and others are found stalking the Mannoroc Coven in Desolace and defending the portals there. Others still stalk Felfire Hill in the Ashenvale forest, keeping watch on the Infernals there and relaying their Masters insidious plans.
During an invasion, the Fel Guard are the first real demons to be brought through and serve as the main bulk of the attack force. The first four Fel Guard to set foot on Azeroth (Besides the Houndsmaster Hakkar) served as Bodyguard to Queen Azshara.
A Fel Guard is an incredibly powerful demon and can only be bound to those who dedicate their studies to demonology, though make excellent bodyguards and are almost like a badge of office to the most powerful demonologists. Indeed, my Felguard, Thootom, is rarely from my side.
Closer to constructs than demons technically, mindless but incredibly powerful, the Infernal is the embodiment of brute force. A burning mass of rock and demonic energy, an infernal is a raging inferno of concentrated power, and they are insidiously difficult to bind. Few Warlocks utilise the power of the Infernal, which is a dire shame considering the sheer power that can be utilised, though it must be stressed that utilising such a power is highly risky as the demon is not bound through the usual methods, instead it is called down then chained and the demon is entirely capable of breaking such chains at any moment; though the sheer destruction the demon can grant its master is not something to be sneered at.
Infernals stalk the Felfire Hill and the barren wastes of southern Desolace, but are equally as common in Hellfire Peninsula where Inferno Storms are commonplace, in which Infernals literally fall from the sky, incinerating their local surroundings before rising to their feet to smash anything that isnt flammable. Though their power soon fades, entire cities have been destroyed before the Infernal crumbles.
The summoning and binding of an Infernal, as stated, is incredibly difficult, requiring an Inferno Stone rather than the usual Soul Shard to call the creature from the Nether. During this summoning, it is temporarily bound to the Warlocks will, but will soon break free and must either be rebound, or banished back to the Nether.
The original Infernals were thought to be created by Nathrezim, estranged cousins of the Doomlords, but since have been created within the Deathforges of Shadowmoon Valley by Mo'arg and Gan'arg. During the first invasion, and since, Infernals are used as elite shock troops, plummeting into enemy lines where the impact is almost as lethal as the onslaught that follows.
Very little exists to be said of the Abyssals as they are pretty much identical to Infernals in application and appearance. The differences being that Abyssals are built of searing fire as opposed to fel fire. They are constructs built from the searing core at the deepest depths of the Twisting Nether.
The Abyssal is a far rarer sight, though the places in which they are seen certainly hints at their nature. A select few are found within the Arcatraz of Tempest Keep, likely captured by the Naaru. The demonic presence at the top of Karazhan (Home of the Last Guardian, the Late Medivh), Prince Melchezzar, has the ability to call upon abyssals from the Twisting Nether. Lasty, the being Supremus within the depths of the Black Temple, and another Abyssal known as Kraator hint at the idea of Abyssals being stronger, more powerful (perhaps even aged) Infernals.
Of Ered'Ruin, or Doomguards:
Winged, incredibly intelligent, and with strength rivalling that even of an Infernal, the Doomguard is a potent foe indeed capable of bringing utter ruination to any area unfortunate enough to be visited by one. Within the Legion and Demonic society, Doomguards are shock troops, flying above the Fel Guard and bringing swift death wherever it is required. Sightings during the first invasion of Doomguard whipping Fel Guard would also hint that the Ered'ruin are higher in rank, possibly similar to Captains or Sergeants. .
It should be stated that, like the Infernal, the Doomguard is not summoned for a Warlocks use by the usual means and instead by one of the following methods:
The Curse of Doom is the first method available to most Warlocks. A potent curse, should it kill its victim, there is a chance the death will attract the attention of a Doomguard, at which point, the Doomguard must be vanquished or bound to the Warlocks will.
The second method is a little more sinister, though far more direct. In the Ritual of Doom, the Warlock and a group of aides call forth a Doomguard, and each member of the group falls victim to a diabolical game of roulette; upon summoning, the Doomguard will claim the life of one of the ritualists instantly.
Once a Doomguard is successfully summoned and bound, the Warlock has a powerful ally, though the demon will undoubtedly eventually break free, either to be rebound or swiftly slain.
There is little to say in the way of fiends, as they are very closely related to Doomguards and serve many of the same functions within Demonic society and military structure. I include them here for identification purposes only.
Very similar in appearance also, varying only in that they bear a face upon their chests and have six horns. The only difference within Military structure between Fiends and Doomguards is that whereas Doomguards lead units of Felguard, the Fiends lead Illidari and act as slavemasters for the Broken.
Inhabiting many of the dark corners of Azeroth, from the Hatefury Satyr of north eastern Desolace, to the camps of Satyr in the Ashenvale forest and ruins of Azshara, to the groups found in the depths of Dire Maul and Maraudon.
Satyr are bipedal and goat-like humanoids, created by corrupting Kaldorei, and are often the first signs of demonic incursion. Where the Satyr arrive, soon more demons follow, and because of this, as soon as Satyr presence is discovered, a purge is usually attempted.
During the first invasion of Azeroth, Lord Xavius, Advisor to Queen Azshara, became the first Satyr as a gift from Sargeras. Xavius in turn corrupted many of his 'favoured' Highborn, creating the first of the satyr.
Satyr society is built upon fear and cunning, each Satyr fulfilling a certain role within the society but constant feuding and plotting is commonplace. Such an internal strife is accepted and even respected by the Satyr who plot and scheme against their superiors, vying for the higher places within society . As such, those few at the top are often well protected and are accustomed to assassination attempts.
Satyrs, more often than most other demonic societies, utilise imp slaves, both as simple servants and as meat-shields to defend against potential assassins .
Unfortunately, because of this internal conflict, Satyr are often well defended and well prepared, so ousting a Satyr infection is nowhere near as simple as it sounds.
Of Nathrezim, or Dreadlords:
Vampiric, powerful and incredibly intelligent, Dreadlords prefer to work from the shadows, corrupting slowly, as opposed to assaulting their enemies directly. Deep within the bowels of Azeroth, the insidious Dreadlords lead their cults in all manner of dark ritual and activity. From Lord Banehollow of Jaednar (in Felwood, an area renowned for being plagued by demonic activity ) to Balnazzar (who had secretly been leading the Scarlet Crusade) to Lord Varimathas - Lady Sylvanas personal advisor - (I refer to them with the Lordly title merely out of respect not for any allegiance on my part), the Dreadlords influence is felt wherever there is any form of Fel activity. The Dreadlords influence may be felt, though it is rarely ever seen, Lord Banehollow, for example, hides away deep within the bowels of the Shadow Hold in Jaednar where he plots against his Dreadlord brethren, especially his brother, Lord Hel'nurath the Demonic Overlord of the plane of Xoroth.
As a point of note, during the first invasion of Azeroth, a small contingent of Nathrezim were seen dabbling with necromancy. This is likely the first time the idea of raising the dead to fight for them was suggested. Of course, this idea would later expand thousandfold ad the Scourge would be created.
During my initial research, I encountered no literature on these creatures, and did not know them to exist until I stumbled across one in Hellfire, then many more in Shadowmoon. As such, information here is speculation from what I have observed in the field and have garnered from the Hologram Emitters aboard the Exodar.
These gigantic six-armed females appear to be priestesses of the Legion and within the Legion itself act as Military Chaplains, serving only to evangelise Sargeras' dread vision to the universe. Their extreme charisma makes them the driving force of the Legion. 
In the field, they weild terrible swords (between two and four has been commonly seen) and attack viciously without remorse. Their very presence seems to inspire nearby members of the Burning Legion making them seem as avatars of Sargeras' vision. They go by names such as Doomsinger or Death Whisperer and have been sighted in Shadowmoon Valley and Hellfire Peninsula.
As gnomes are to the Alliance, the Mo'arg are to the Legion and equally as vile. Immense hulks of sinewy flesh and felsteel, the Mo'arg are tinkerers, scientists and engineers, often "enhancing" their own bodies almost to the stage of looking not dissimilar to the stitched horrors of the Scourge though very much more intelligent. The Exodar states them as: ""Brutish... ...these iron-stitched horrors are exceedingly brilliant engineers and forge-smiths. Their black iron constructs have sown death across a thousand worlds."  And indeed, it is their hands that have created monstrosities such as the Doomwalker that guards the Black Temple and the Fel Reavers that stalk Hellfire. They are also attributed to the creation of Infernals within the Shadowmoon Deathforges and the portals of the Legion.
Of Man'ari Eredar:
In Eredun (commonly referred to as Demonic), Man'ari translates as: "horrifically wrong, something twisted and unnatural and defiled..."  and indeed by this definition, Fel Orcs, Naga and Satyr would be described as Man'ari. However, it is most commonly used to refer to demonic Eredar. Just as one faction of Eredar escaped Draenor and renamed themselves Draenei, so those who have succumbed to the Legion are known as the Man'ari Eredar.
Originally, it is stated that Sargeras visitted the three great Eredar leaders, Kil'jaeden, Archimonde, and Velen, and promised them immortality and power. Velen declined and now leads the Draenei. Kil'Jaeden and Archimonde are now powerful leaders within the Legion.
The Eredar primarily fulfil the roll of battlefield commanders and warlocks, passing on the will of their greater masters and sowing destruction and discord wherever their influence can reach. Some Eredar have grown to great power, such as Prince Melchezzar who currently resides atop Karazhan, although his presence and ultimate aims with the tower are still a mystery.
Corrupted Eredar, the Wrathguard have long tails, reversed knees, vicious horns and a formidable suit of armour. The Wrathguard act as Honour Guard for the Eredar leaders. They are cunning and precise and their skill with weaponry and their combat abilities should not be underestimated , least of all due to their ability to create streams of flame with which they scorch possible assailants.
Of Annihilan, or Pit Lords:
Great engines of destruction driven by rage and bloodlust. Their bodies resemble massive lizards, with four powerful limbs, a tail, and wings sprouting from their backs (the wings are vestigial and occasionally used for defense - even with eredar magic, Pit Lords are far too heavy to fly). Their upper torsos are more humanoid, with a large chest, two arms and a large horned head. Their mouths are filled with rows of teeth, their eyes are on fire and even their tongues are studded with fangs. The preferred weapon of the Pit Lords is a massive double-bladed sword. Though they may look fat and lazy, they are unimaginably powerful.
Serving their master Mannoroth, they are fanaticly loyal and will stop at nothing to ensure that the will of the Burning Legion is upheld, acting as Generals and Ground Commanders who are more than willing to join in the fray.
Currently Magtheridon, a powerful Pit Lord, is being held captive at the bottom of Hellfire Citadel by the Fel Orcs there.
Of Fel Reavers:
Constructed by the Mo'arg, the Fel Reavers are enormous constructs of felsteel powered by a fel fire core. They are immensely powerful and are used as Sentinels, patrolling the watses of Hellfire Penninsula. They are incredibly difficult to destroy and the ruins of one such success is hailed as an almighty success by the Horde, and a similar ruin is used by the Humans of Stormwind as a resource, scrap metal plundered and sent home to be melted down for use in weaponry.
One such Fel Reaver, known as the Doomwalker, stalks the area outside the Black Temple and was sent by Kil'Jaeden to assault the gates. Any groups wishing the enter the temple, must somehow first get past this monstrosity of fire and steel who's every step and voice can be heard resounding around the entire of Shadowmoon Valley.
It is a foolish man indeed who dares pit himself against a Fel Reaver.
Of Fel Orcs:
Mannoroth's blood was used to forge a blood-pact with the corrupted orcs on Draenor, making their corruption permanent - only one clan, the Frostwolves, escaped. During the Burning Legion's second invasion, Thrall and Grom Hellscream defeated Mannoroth, and broke the blood pact he had forged and thus freed the Orcs from demonic corruption. Hellscream did not survive the encounter.
Those who did not escape remained in Outland and served Mannoroth, becoming the Fel Orcs. After their consumption of the Blood of Mannoroth, a heavy blood haze fell over the orcs, forcing them into a crazed bloodlust. The most prominent side effect of this blood haze is a deep red discoloration of the flesh and insane desire for battle and violence. Fel orcs are also more powerful than regular orcs.
The clans of Fel Orcs now currently man the Hellfire Citadel, where they rule over their once master Magtheridon after Illidan imprisoned him, where they now serve the Betrayer under direct command of the Fel Orc, Kargath Bladefist.
Of Beholders (Floating Eyes):
A ball of tainted flesh inset with many eyes with multiple tentacles drifting through the air; the Beholder is a being of sheer terror and can bring a sense of doom to any they gaze upon. These intelligent creatures are far less loyal to the legion than most other demonic races, as exemplified by two of their possible leaders, Broggok of the Illidari and Shirrak the Dead Watcher of the Auchenai. However, just as likely as rogue are those who serve the Legion, such as those who watch over the Death's Door in Blades Edge. Beholders are the first demonic species to show offspring, smaller, single-eyed creatures.
Of Void Terrors:
Very little is known of these dog-like beings. Two headed, each head with a single eye and a double crest of chitinous horns with a bristling array of eyes behind the twin heads. Their scaled hides are incredibly thick and two larger creatures of the species can be found within Blackrock Spire and the demon Immol'Thar imprisoned within the ruins of Eldre'Thalas.
Despite the bristling array of eyes it has, a Void Terror also has the capability to summon floating eyes similar to the Eye of Kilrogg. However, unlike this common Warlock spell, the Void Terrors eyes are capable of damaging foes. It is unknown how these beings are used within the forces of the Burning Legion.
And so ends my study into the Fel. I hope these words may be of assistance to whomsoever chooses to read them. As a Nethermancer myself, a Master Demonologist and the Fel Warden, this research has been most intriguing and has piqued my interest in numerous other matters.
-  Harnessing Shadows
-  Eredar Explained by Zenith Stormreaver
-  The Journal of Pagatha De LAurent
-  Hologram Emitters aboard the Exodar
- All other information garnered from personal experience on my travels across Azeroth and the shattered ruins of Draenor.
- The Scryers, and especially Seer Librarians Sarai and Kaliim Ravensinger, for access to the Library and its vaults.
- Kruellagh De LAurent for information on Pagatha and her Journal.
- Drazial, Edomer and Kulerrta for continued support and protection during my research.
- My dearest Celessia for being there always.