The Illidari was the faction that Illidan Stormrage created after becoming the Lord of Outland. It consisted of his forces, including blood elf and night elf demon hunters, Coilskar Naga, Ashtongue Broken, and Shivarra. The faction was practically disbanded after the death of Illidan, and the capture of his demon hunters, at the Black Temple, but reformed ten years later when the demon hunters were released from the Vault of the Wardens by Maiev Shadowsong, in order to combat the Burning Legion's third invasion of Azeroth. In the absence of Illidan, one prominent demon hunter led the Illidari and had to choose whether they wanted Kayn Sunfury or Altruis the Sufferer as their lieutenant. The Fel Hammer on Mardum became the headquarters of the Illidari, since Akama would no longer allow them in the Black Temple.
The Illidari is technically a playable faction, since demon hunters start off serving Illidan. The Illidari is also the class order for demon hunters.