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.