Peter of Pomfret is a madman who makes prophecies in the street. One of his prophecies gets him in trouble with the authorities. He predicts that King John will give away his crown by the next Ascension Day.
King John is not happy about this at all, so he condemns Peter to death. As it turns out, King John does give up his crown on Ascension Day—when he hands it over to Cardinal Pandolf, who hands it back again. King John realizes that maybe Peter's prophecy was harmless after all. Still, he doesn't make any effort to undo his previous order. We can only assume that Peter of Pomfret meets his end at the end of a rope.