In Norse mythology, Heimdallr is a god who keeps watch for invaders and the onset of Ragnarök from his dwelling Himinbjörg, where the burning rainbow bridge Bifröst meets the sky. He is attested as possessing foreknowledge and keen senses, particularly eyesight and hearing. In keeping with his guard duties, Heimdallr possesses the resounding horn Gjallarhorn and the golden-maned horse Gulltoppr, along with a store of mead at his dwelling. He is the son of the Nine Mothers, and he is said to be the originator of social classes among humanity. Other notable stories include the recovery of Freyja's treasured possession Brísingamen while doing battle in the shape of a seal with Loki. The antagonistic relationship between Heimdallr and Loki is notable, as they are foretold to kill one another during the events of Ragnarök. In other places of Norse lore, Heimdallr is additionally referred to as Rig, Hallinskiði, Gullintanni, and Vindlér or Vindhlér. Heimdallr is attested in the Poetic Edda, compiled in the 13th century from earlier traditional material; in the Prose Edda and Heimskringla, both written in the 13th century by Snorri Sturluson; in the poetry of skalds; and on an Old Norse runic inscription found in England. Two lines of an otherwise lost poem about the god, Heimdalargaldr, survive. Due to the problematic and enigmatic nature of these attestations, scholars have produced various theories about the nature of the god, including his apparent relation to rams, that he may be a personification of or connected to the world tree Yggdrasil, and potential Indo-European cognates.