Yssarile: The Daemon-King whose followers built Ghul.
He fought with what was believed to be one of the first three of the four main powers of Chaos. Due to references to a 'foul sorcerer' it could easily have been Tzeentch. The war lasted for one billion years before he was defeated by the power he was trying to usurp.
Source: Hereticus by Dan Abnett, Eisenhorn by Dan Abnett
Submitter: IBBoard
Hive World Terra