The banner of the Illidari
The Illidari is the faction of Illidan Stormrage, the Lord of Outland. This group is also known as Illidan's Forces or Illidan's Army. It consists of various factions that have banded together under Illidan's rule. They were one of the prime enemies of the Alliance and Horde in Outland, the other being the Burning Legion. At its prime, The Betrayer commanded vast armies throughout Outland, and even a few select groups on Azeroth; there were only a few places in the shattered world that are not inhabited by his legions.
After their horrific defeat at the hands of the Alliance and Horde, the surviving Illidari either were imprisoned or found their own paths.
Illidan brought some night elf demon hunters from Azeroth with him during his campaigns in Outland. Some young blood elves also trained under Illidan. During the raid on Black Temple, Illidan deployed his demon hunters on a suicide mission to the titanic prison world for the Burning Legion. After his defeat, Maiev Shadowsong imprisoned his surviving followers within the Vault of the Wardens.
The main group of Illidari that are explicitly labeled as such seems to serve as an elite group within the overall faction. This group was composed mostly by demons of many species, particularly Dread Lords, Succubi, Terrorguards, Felguards and Satyr, former minions of the Burning Legion who switched their allegiance after the defeat of Magtheridon.
Unbeknownst to the Blood Elves, Illidan kept a massive source of untapped arcane energy, similar to the Well of Eternity, within the Shrine of Lost Souls in Karabor. The demon hunter used this energy to bargain with the demons.
By feeding upon this place's power, they were broken free of their addiction to fel magic, and thus their connection to the legion. After Illidan's defeat, the surviving demons broke off from the Illidari and became known as The Unbound.
When Illidan invaded Outland, Magtheridon's primary non-demonic forces were fel orc, corrupted by his own blood. Led by Kargath Bladefist, the fel orcs joined the Betrayer's service and primarily man the battlements of Hellfire Citadel. In addition, members of the Shadowmoon clan were found on the Ata'mal Terrace of Illidan's Black Temple, and the survivors of the Dragonmaw clan - under their aged chieftain, Zuluhed the Whacked — capture and take control of nether drakes for service in Illidan's armies.
Kael'thas's Sin'dorei - Illidan was briefly captured by his long-time jailor and nemesis, Maiev Shadowsong, shortly after arriving in Outland. It was primarily due to the intervention of Kael'thas Sunstrider and his blood elves that allowed the naga under Lady Vashj to free him. The blood elves under Kael's command, having been scorned by the Alliance, swore allegiance to Illidan and aided him in toppling Magtheridon. The Betrayer's most trusted legion of blood elf warriors is known as the Crimson Sigil; they are the highest commanders of the Illidari outside of the Black Temple itself. Though Kael betrayed Illidan in favor of Kil'jaeden, some blood elves remained loyal to the Betrayer, such as the demon hunter initiates under Varedis, and the Eclipsion - Blood elves under Grand Commander Ruusk.
Calling in some old Highborne debts from ten millennia earlier, Illidan raised the naga from their undersea lairs in order to aid him in obtaining the Eye of Sargeras, as well as preventing Maiev from following him. Under the direction of Lady Vashj - The naga followed Illidan into Outland where they were instrumental in both obtaining the aid of the blood elves and laying siege to Magtheridon's fortress. Prior to Vashj's death, her naga controlled the watery areas of Outland, including much of Zangarmarsh.
The Betrayer obtained the aid of Akama and his Broken draenei sometime before the siege of the Black Temple. The Broken proved to be proficient assassins and scouts, and were able to allow Illidan and his army to reach Magtheridon. Though only the Broken of Akama's tribe served Illidan willingly (though the same cannot be said of Akama himself), Illidan's forces in Hellfire Peninsula and Zangarmarsh have enslaved Broken for menial labor. A few corrupted tribes of Lost Ones also served Illidan and his allies while other tibes seem to have been enslaved. Akama and the Ashtongue eventually betrayed Illidan, seemingly killing him and ending his rule over outland. The Ashtongue then took control of Karabor, vowing to eventually cleanse it of all its corruption.