Certain monsters with shells will become withdrawn if injured sufficiently, retreating into their shells for safety. In this state they refrain from attacking and attempt to recover their health. This gives them a +10 boost to AC and boosts their HP regeneration rate, but makes them stationary and unaggressive. They will eventually return to combat if allowed to recover, but it is generally easy to finish them off before this happens.
Monsters that can use this ability:
This monster ability was removed in 0.19.