Loki is a god in Norse mythology. According to some sources, Loki is the son of Fárbauti (a jötunn) and Laufey (mentioned as a goddess), and the brother of Helblindi and Býleistr. Loki is married to Sigyn and they have two sons, Narfi or Nari and Váli. By the jötunn Angrboða, Loki is the father of Hel, the wolf Fenrir, and the world serpent Jörmungandr. In the form of a mare, Loki was impregnated by the stallion Svaðilfari and gave birth to the eight-legged horse Sleipnir. Loki's relation with the gods varies by source; he sometimes assists the gods and sometimes behaves maliciously towards them. Loki is a shape shifter and in separate incidents 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 eventually, Odin's specially engendered son Váli binds Loki with the entrails of one of his sons; in the Prose Edda, this son, Nari or Narfi, is killed by another son of Loki who is also called Váli. In both the Prose Edda and the Poetic 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. Loki is referred to in the Poetic Edda, compiled in the 13th century from earlier traditional sources; the Prose Edda and Heimskringla, written in the 13th century by Snorri Sturluson; the Norwegian Rune Poems, in the poetry of skalds, and in Scandinavian folklore. Loki may be depicted on the Snaptun Stone, the Kirkby Stephen Stone, and the Gosforth Cross. Scholars have debated Loki's origins and role in Norse mythology, which some have described as that of a trickster god. Loki has been depicted in or referenced in a variety of media in modern popular culture.