There is 1 Dead Ghost to be found in The Archon's Keep. They can be collected during any patrol or mission. Each Ghost can only be collected once per account.
Ghost 1[edit]
Head out of the Archon's Forge to this location across from the entrance of the forge, in The Archon's Keep. Go through narrow exit toward the map boundary.
Jump over the ledge to the small outcropping below. You will be out of bounds, so you need to immediately turn and face the cliff-face.
There is a small opening above the waterline to your right. Jump into that opening and head inside.
Inside the cave, go to the left and look behind the rocks there. You will find the Dead Ghost on the floor.
