Fel

Fel is a highly destructive form of magic well known for being used by members of the Burning Legion and demons. However, to those whom study magic it is know as the magic of disorder; demonic, chaotic and extremely volatile it is stated to be a brutal and extremely addictive energy that is fueled by drawing life from living beings. Though energy cannot be broken down or created and life energies are not some created resource to serve as fuel for energy. Fel magic is a spiritual magic that life force (Or spirit) is transformed into with something such as the Life Tap ability. Spirit is a life-giving force that binds all things in existence as one, sacrificing its life.

Sacrificial magic such as Fel Magic was considered the greatest violation of life by the titans (living worlds) and still to this day is. Titans had hounds, instantly attuned to punish those who delved into such sorcery and police the use of arcane magic-- The magic of order.

Together however, the forces of disorder and order govern the cosmic systems of the physical universe. As the force of disorder, the highly destructive fel magic does affect systems. Indeed concentrated fel energy can cause systems to go haywire.

Also as the opposing magic to arcane Fe Magic is wildly rumored to consume such magic and use it as fuel. However Arcane magic being the magic or order can also be used to counter Fel Magic if used correctly. This is because they are opposite ends of the same spectrum.

As the magic of disorder Fel is often used by the wielder, or its effects on the victim, frequently results in an alteration of the individual, often leading to corruption. All demons carry the taint of Fel Magic within their very blood, allowing them to spread greater evil.

Fel Poison
Fel-poisoned creatures are another threat, appearing in places where the Burning Legion is strongest not even dragons are immune to the corruption of Fel Magic. When a creature with a natural poison attack is imbued with Fel energy, its venom becomes nearly three times more dangerous and even more difficult to resist.

Fel Magic
Spells with the fel descriptor are very difficult to resist. Some arcanists can can fel spells, perhaps after drinking demons blood. Casting a fel spell is described by many spell casters as a euphoric experience. If Arcane Magic is an addiction, fel magic is the embodiment of the drug in its purest form.

Demons often tempt spell casters by offering small amounts of their blood in exchange for services. If the recipient drinks the demon’s blood within 24 hours of the offering, the next arcane spell they cast will be a fel spell. However, one must keep in mind that Fel Blood is just as highly addictive as well as corrupting. Anyone who drinks it has a chance of becoming dependent on Fel Blood, and unless the person continues to drink it daily, they will suffer withdrawal symptoms. They have a chance of breaking the addiction seven days after their first drink of the Fel Blood.

Fel Weapons
When the demons brought death and damnation to the world, they also carried their own brand of magic to the world. Their blood had magical properties; it addicted arcane magic users and could be used to create evil weapons. When a weapon is forged in demon blood, it becomes a fel weapon. It glows with a foul, Yellow-green fire and deals extra damage to good-aligned opponents being mainly light users.

Effects on beings
Fel corruption can manifest with positive effects in which case it can be considered a gift or a blessing, like that of the Eredar, or with negative ones in which case it can be considered a curse. Physical transformation normally ensues, such as a change to an individual’s eyes or skin color or heavy genetic mutations such as growth of demonic traits like spikes scales and horns, Negative corruption tends to lead to physical or mental deterioration, as what happened with the Draenei that turned into Broken and Lost Ones, or as what happened with boars which turned into Hellboars.

Either way, corruption gives the individual increased strength, more magical potential and can lead to transformation into a demon. Warlocks with long exposure to or with extensive use of fel magic age faster than normal, and become phyically weaker over time. Fel seems to often cloud one’s judgement, or at least heavily amplify their negative traits such as violence and impatience, while snuffing out the positive ones such as empathy, allowing one to make decisions they would never make before.

As the existence of non-sentient demons suggests, a creature’s wishes are irrelevant to the process. Willing or not, a creature can be changed into a demon.

Demons
Demon souls are attached to the Twisting Nether and as such even if a demon dies in the physical universetheit spirit will return to the Nether where they will reform once again. To truly destroy a demon, the creature must be killed in the Neather itself, in places where it bleeds into the mortal worlds, or in areas saturated in inflicting suffering, spreading corruption and destroying all that is good. Mortal beings can be turned into demons themselves by being infused with Fel energy.

Eredar
Sargeras infused the Eredar with Fel Energy, turning them into demons now called Man’ari, Eredar and found quick use for his Fel-corrupted converts. The Eredar settled in as commanders within the Burning Legion. Under the Eredar’s Leadership, the ranks of the Burning Legion swelled with new demonic races gathered from the Great Dark Beyond.

The Old Horde brought the Red mist upon the Draenei, causing them to mutate and devolve into the Broken and then further into Lost Ones. Their warped bodies bear only a rough resemblance to the proud Draenei they one were and quite a few have lost their grip on sanity.

Orcs
Gul’dan was the first Orc warlock and is considered one of the most powerful mortal warlocks to have ever existed. He taught other young Orcs the Arcane arts and strove to eradicate the Orcs shamanistic traditions. Kul’jaeden, seeking to tighten his hold over the Orcs, helped Gul’dan found the Shadow Council, a secretive sect that manipulated the clans and spread the use of Fel Magic through Draenor. As more and more Orcs began to wield warlock magics, the gentle fields and steams of Draenor began to blacken and fade. From every account, the demonic energies, used so freely and with no heed fiven as to their coat, leeched all that was wholesome and life-giving from the world of Draenor.

The originally brown orcs changed from under the influence of Mannoroth’s blood, their skin became various sades of green and they began experiencing lusts for blood.

Orcs that kept drinking the pit lords blood became Chaos Orcs and Fel Orcs.

Elves
Blood Elves are High Elves who had temporarily turned to absorbing demonic magic after the loss of the Sunwell and had subsequently became addicted to the fel energies. From the most pious of priests or most outdoorsy of Farstriders, and High-Elf in Quel'Thalas or Outland following the Third War chances are they have been around Fel energies, which made their eyes turn green. Keal'thas Sunstrider’s most loyal followers were rewarded by Kul'jaeden with the gift of being allowed to gorge themselves on demonic blood for days for remaining in Outland, even as many of his blood elves retreated through the Dark Portal. These are known as Felblood elves.

Statyrs have been transformed into their current shapes by inheriting the demonic curse that originated with Xavius.

Titans
Titans are natually very vulnerable to Fel Magic. When Sargeras was infused and deformed by the explosion of Fel energy caused by the shattering of Mardum, the Pantheon could not overcome Sargeras’s fel-fuled might. The fallen titan decimated the Pantheon members by summoning a massive fel storm that would consume their bodies and souls alike, though Norfannon managed to save their souls and made them flee towards Azeroth where they would vanish.

Void Revenants
Void revenants are a race of parasitic void-beings that tear holes through the planes of reality, consuming and absorbing the energies and beings within. Xhul'horac was twisted and fed overwhelming fel energy by alternate Gul'dan and wanted to use his powers to tear open the portals through which he planned to rally the forces of the Burning Legion.

Demon hunters
Demon hunters, uphold a dark legacy, one that frightens their allies and enemies alike. The Illidari embrace fel and chaotic magics—energies that have long threatened the world of Azeroth—believing them necessary to challenge the Burning Legion. Wielding the powers of demons they've slain, they develop demonic features that incite revulsion and dread in fellow elves.

Demon hunters mark their bodies with arcane runes to keep their fel forces in check.

Druids
Some druids have manipulated fel energy to enhance their sense with the wild.

Mages
Some mages gave up the arcane to begin a personal quest to understand demons and their fel energies, like Daio the Decrepit, effectively becoming warlocks. Some warlocks are also named fel magus such as Fel Magus Verondis and Fel Magus Kel'rothek.

Dalaran attempts to stifle the studies of fel magics and necromancy. Despite the superior power of fel magic which threatened to overshadow established practices, students were taught about practicality over what is achievable. Fel's efficiency is commonly balked for being unstable and "evil". The official teachings of the Kirin Tor are that demonology is to be eschewed, avoided, and abjured. Any attempt to summon demons are to be found out and stopped at once, and those involved are to be expelled, or worse.

Warlocks
Warlocks are magical practitioners that seek to understand darker, fel-based magic, and its destructive spells. While many warlocks willingly follow the Burning Legion, there are those who work against it, using their magic to fight against evil. Warlocks have proven themselves to be powerful allies-as well as powerful foes. Warlocks convert life into fel, draining the victim as a source of power.

Sources of information
https://wow.gamepedia.com/Fel