Ahriman

Ahriman, Sorcerer of the Red Cyclops, was the Chief Librarian of the Thousand Sons Legion, and - after his Primarch, Magnus the Red - the most powerful of all Chaos sorcerers.

It was he, leading a cabal of powerful sorcerers, who cast The Rubric of Ahriman that sealed the Legion's suits of power armour and turned their bodies to dust. Although his plan was to save the Legion from the Tzeentch-caused mutations affecting them, Ahriman was responsible for effectively destroying the Legion, and was subsequently exiled from the Planet of Sorcerers that had been claimed as the Thousand Sons' new homeworld by the daemon-Primarch Magnus the Red.

Ahriman's lust for power has no equal, and he does not acknowledge Chaos as his master. Instead, he is constantly searching for any powerful artifacts or forbidden knowledge that would allow him to fully master the way of the sorcerer.

Ahriman's main goal throughout the 41st millennium is to gain access to the vast repository of knowledge known as the Black Library, which is hidden somewhere in the Eldar Webway.

During the Eye of Terror campaign, Ahriman's search for the Black Library came one step closer as he managed to find out how to get into the Eldar Webway; however, with the rather insufficient forces available to him, a combined force of Eldar Aspect Warriors and Harlequins forced him out. In any case, the Thousand Sons are still able to use the Webway in small numbers - this is probably how the Chosen of Ahriman have now acquired the ability to "teleport" Thousand Sons troops around the battlefields.