When you are in the keep underground, you will enter a room that looks like it has hanging meat in the center.
At the end of the room is a pool of water. On one side is the rosary, the other is a weight.
On the weight side, just cut the rope. The weight will drop, lifting the rosary out of the water.
Collect it to ping the cheev, and now be able to listen to dead soldiers spirits who sometimes have clues for ammo and weapon locations.