They're all dead in Valhalla
Valhalla's a stable time loop where the characters die violently over and over again. One-Eye's prophetic visions are just him remembering fragments of past iterations of events. It's also possible that he's the only one who remembers how events play out, but can't tell the others because he's mute, or just doesn't care anymore.
One-Eye is Odin
One-Eye isn't human but rather the god Odin under the guise of a mortal. In Norse mythology, Odin is associated with war, strength, knowledge and prophecy among other things. One-Eye's abnormal fighting capabilities, his apparent knowledge of the world around him and his strange bouts of clairvoyance make him rather similar to Odin. Not to mention, one story in Norse mythology tells of how Odin traded one of his eyes for wisdom and foresight - an action mirrored by One-Eye's personage.
The movie chronicles the Christian crusaders as they get sent to their doom by the Norse gods. One-Eye, who's either meant to be Odin or Vidar, is leading the crusaders into a terrifying, foreign land where they'll all perish. The crusaders are being punished for harming the followers of the Old gods in their introductory scene.