System Shock

System shock is an effect that occurs in certain situations that causes a creature to lose 10% of its maximum hit points.

Causes

• When a familiar dies and its corpse decays or is eaten. This does NOT occur until the corpse is gone, so resurrecting the familiar or preserving the corpse will prevent system shock. Similarly, if the familiar is turned to stone, system shock will not occur unless/until the statue is shattered. This happens only with familiars, not other pets.
• If a creature that is possessed dies, the possessor suffers system shock. This includes creatures that are possessed by using the Possess spell, and also a Wizard's Eye.
• System shock will also occur when a summoned creature is possessed and the summon duration expires, except for a Wizard's Eye (where this is expected).
• If a creature is polymorphed and loses all of its hit points in the new form, it will return to its original form and then suffer system shock. (Unless it had Unchanging, in which case it will just die.)