Sindri Myr was the Chaos Sorcerer in the original Dawn of War game. He is shown throughout the game to be a second-in-command of the Chaos Lord Bale, until Sindri betrays him, and Bale is killed by captain Gabriel Angelos.
Sindri is of the Alpha Legion Chaos Space Marines and seeks a daemonic artifact known as the Maledictum, which houses an ancient Greater Daemon of Khorne, the one that Chapter Master Kyras entreats with in Dawn of War 2: Retribution. Sindri is instrumental in the corrupting of Librarian Isador Akios of the Blood Ravens, turning the Librarian against his brothers. Sindri eventually betrays Isador as well, and takes the Maledictum for himself.
He ascends to become a Daemon Prince in the last level of the campaign, and is subsequently slain by Gabriel Angelos with his Daemonhammer God-Splitter. However, Angelos then proceeds to destroy the Maledictum, and unwittingly unleashes the daemon which would go on to plague the Aurelia sub-sector in Dawn of War 2: Retribution.
NOTE: Sindri Myr is a much more powerful Daemon Prince than the unit introduced in Winter Assault.