The Burning Legion is the huge demonic army created by the titan Sargeras in the fictional Warcraft universe. Originally a playable race in Warcraft III, the race was changed during play-testing to a set of non-player characters and monsters.
When Sargeras the Dark Titan decided to set the universe aflame after becoming deeply sorrow stricken by evils that manifested from the twisting nether, he created a demonic army to serve that purpose. The Burning Legion was led by Archimonde the Defiler and Kil'jaeden the Deceiver, two Eredar warlocks who possessed immense power.
The two Eredar commanders chose malicious races to help them in their evil work. Archimonde chose the evil Pit Lords and their barbaric leader Mannoroth the Destructor, while Kil'jaeden chose the Nathrezim, the vampiric dreadlords. Among all the dreadlords, his favorite was Tichondrius the Darkener, and Kil'jaeden entrusted him with most of his affairs.
Before their first failure in Azeroth, the Burning Legion destroyed countless planets and killed millions of living things. They are responsible for the destruction of Draenor and the Draenei, as Kil'jaeden made the orcs drink Mannoroth's blood, thereby enacting the Blood Pact and binding their fates to the Legion forever. Crazed by the chaotic energies in Mannoroth's blood, the orcs descended into a frenzy and killed all in their path before entering the world of Azeroth and creating a conflict there.
The Burning Legion have tried to invade the world of Azeroth on two occasions (although it should be noted that both the First War and Second War occurred thanks to their original invasion) ; the first invasion nearly destroyed the world because of the explosion of the Well of Eternity. The second one resulted in the death of Archimonde and the near destruction of the World Tree. Now Kil'jaeden leads the Burning Legion alone without any rivals.
Burning Legion contains hundreds of different demonic creatures who scream for mayhem and havoc. These include:
Felhound: Felhounds are demon hounds used by the Pit Lords to sniff out sources of magic wherever they be found. Felhounds, who feed upon magic itself, are fond of draining the energies of hapless wizards and then ripping their bodies to shreds. A variation of the felhound also exists, known as the Felhunter. These beasts are recognisable by a golden ring on one of their horns. In the "War of the Ancients" trilogy, they are said to be as big as horses.
Dreadlord: The Nathrezim, better known as Dreadlords, are a scheming, cunning race of vampiric demons. Lieutenants of the Legion, they were the manipulators that influenced the Lich King to unleash the Scourge upon Lordaeron. These beings can be traced to the beginning of the Burning Legion, as they corrupted Sargeras. It should be noted lore implies this was not their plan, as Sargeras actually defeats and seals the Nathrezim in "prisons" in the void along with the other demons, but the sheer nefariousness of these creatures left a lasting scar which would eventually lead to their freeing and the initiation of the Burning Crusade.
Eredar Warlock: Born of the Eredar race, warlocks are absolutely corrupt and unimaginably powerful. Their chaos magics have burnt out whole worlds and annihilated countless species over the aeons. Kil'jaeden taught the orcs the secrets of warlock magics, but the orcs could never master the powers of entropy and destruction as well as the wicked Eredar, save for the obvious exception of the Lich King himself. Under Archimonde's command, the warlocks serve as the Legion's tacticians and strategists.
Doom Guard: The fearsome Doom Guard serve as Archimonde's personal escorts. Although they are often called upon to perform a number of duties for the Legion, their loyalty lies with him alone. These monstrous, fiery warriors are nearly immune to magic and can defeat entire armies with their sheer strength alone. It is worth pointing out that in the "War of the Ancients" trilogy, they are no less vulnerable to magic than any other race.
Pit Lord: The Pit Lords who serve under Mannoroth the Destructor are some of the most cruel, barbarous butchers to ever roam the trackless wastes of the Twisting Nether. These hulking engines of hate and death love only to kill and bring sorrow to all living creatures. Fanatically loyal to Mannoroth, the Pit Lords will stop at nothing to further the will of the Legion.
Infernal: These mindless giants of flame and fury are summoned by the warlocks' Inferno spell. Falling to earth as green, molten meteorites, infernals exist only to destroy every living thing in their path. Though their lifespans are limited, the mighty Infernals have been known to destroy entire cities before their energies dissipated back into the Great Dark Beyond.
Mo'arg (known as the "Felguard" in Warcraft III): A holographic image of the legions members within the Draenei capitol The Exodar, reveals that this race makes up the majority of its engineers, and infantry positions, with smaller members known as Gan'arg serving under their larger relatives. It's plausible that these creatures are similar in origin to the orcs (subjugated against their will and made into killing machines) albeit on a more permanent level.