Croag was a member of Yawgmoth's Inner circle.
He was the overseer of Rath and was the one that, along with Davvol, brought Dominaria populations to Rath.
Croag was seriously wounded by Kreig of Keld in a battle to find Gatha and his Bloodlines progress. He took a long time to recover from that wound, during which Davvol devised ways to get rid of him. When Croag had recovered, he defeated Davvol's negator and trapped Davvol in the Death Pits of Rath of the Stronghold.
General Croag's task was also to find and kill Urza.