Creature in Chains

This card enters the battlefield with 7 prison
counters on it.
Whenever a card enters the battlefield or is exiled remove a prison counter from this card. When
this card has no prison counters, sacrifice it and
summon a Demon Specter creature from your
hand or the command zone.
Kraledemos's chains had 7 locks, each with a different
key. Each was guarded on a different continent...