Crystal is a special grouping composed of stalkers from the Monolith faction that was responsible for the safety of the Monolith itself. The reality is that these Stalkers guard the Control room for the C-Consciousness' creation, The Wish Granter. The group can be found in the Monolith Control Center, and the group of Veteran and Expert ranked Monolithians must be killed for the player to enter the Control room, as the door is scripted to remain locked until their deaths.