Kil'jaeden's palace

The enormous, crystalline palace city of Kil'jaeden is where the rulers of the Burning Legion dwelled. It is located on the ravaged world of Argus, within a city laid out according to complex geomantic rules. Its curved structures are reminiscent of draenei buildings, only far more intricate and beautiful. Outland's draenei town halls were hovels compared with the fantastic structures of this city. There were huge machines for focusing magic, which once provided peace, harmony, and health to an entire world. Now they created a cloud of fear and despair.

At the center of the great city stands the mighty palace, made of crystal corridors forming a dark labyrinth and engraved with runes that glow with evil significance. It is as if the core of all the spells that had once spread light and harmony across the city and the world had been rewritten to create the opposite. Illidan Stormrage managed to project himself astrally to Argus and infiltrate the palace. When he studied the runes within, feelings of rage and despair filled his mind. Even shielded as he was, the spells affected him, filling him with visions of conquest, a lust for domination and destruction, a rage to end all things. Here, written in runes of fire, was the creed of the Burning Legion.

Illidan aimed to open a portal to here from Outland with his Illidari, with the goal of killing the Legion's ruler in a swift, short strike. His attempt was interrupted by the champions from Azeroth and Maiev Shadowsong, and her wardens.

Speculation
Absent in the areas visited on Argus in World of Warcraft: Legion, the location may have been conceptually split into the Seat of the Triumvirate and Antorus, the Burning Throne, as the city described seems to be a fel-corrupted Mac'Aree.