What is the Burning Legion in World of Warcraft

The Burning Legion is the combined might of demonic forces traveling the Twisted Nether, invading worlds and conquering them. Their aim is to have the inhabitants of these worlds join their legion as demons, and conquer more worlds until all falls under their reign.

The tacticians of the Legion are the Nathrezim, the vampiric Dreadlords, with their inifinite schemes and charisma. Leading the forces are the Eredar, most notably Kil'jaeden and (before his destruction) Archimonde. At the very head of the Legion stands the fallen Titan Sargeras, whose hatred and lust for conquest drives the Legion ever forward.