Loki's relation with the gods varies by source; Loki sometimes assists the gods and sometimes behaves in a malicious manner towards them. Loki is a shape shifter and in separate incidents he appears in the form of a salmon, a mare, a fly, and possibly an elderly woman named Þökk (Old Norse 'thanks'). Loki's positive relations with the gods end with his role in engineering the death of the god Baldr and Loki is eventually bound by Váli with the entrails of one of his sons. In both the Poetic Edda and the Prose Edda, the goddess Skaði is responsible for placing a serpent above him while he is bound. The serpent drips venom from above him that Sigyn collects into a bowl; however, she must empty the bowl when it is full, and the venom that drips in the meantime causes Loki to writhe in pain, thereby causing earthquakes. With the onset of Ragnarök, Loki is foretold to slip free from his bonds and to fight against the gods among the forces of the jötnar, at which time he will encounter the god Heimdallr and the two will slay each other.