Oversoul

The Oversoul is a mysterious entity found in Crota's End. It can be seen from outside Crota's fortress, and from within. During the fight with Crota, the Oversoul acts as Crota's Enrage weapon; if a Guardian dies, or Crota becomes Enraged after 10 minutes, the Oversoul will activate. Guardians have ten seconds to damage the Oversoul or else they die.