Geist Ambusher can be found in the Pit of Saron in Icecrown Citadel. They spawn only after your party kills Forgemaster Garfrost. After which, on the path between Garfrost and the hill leading up to the cave, a group of freed slaves will annouce they plan on attacking the Scourgelord's location. These ambushers jump off a ledge and attack the group of freed slaves.