Falysion, Frenzied Mind
From Destinypedia, the Destiny wiki
Falysion will spawn after a Public Event has ended in The Cistern near a small island. Players will simply need to damage it consistently until dead, in which a message will appearing stating that "Falysion, Frenzied Mind, has been vanquished." If not damaged, Falysion will linger for a short period of time before vanishing.
List of appearances