Death grip crystals were Void constructs created by Amon. During the End War, his forces were using the energies from a xel'naga temple in order to create a voidshade hybrid. The death grip crystals were deployed near the void slivers in order to defend it.
Death grip crystals attack nearby enemy units with an energy beam that completely disables them for a short duration. When the duration ends, targeted units are killed instantly. Destroying the crystal will free any units caught in its grip.