Eyepatch of Power: Hagen is one-eyed since his youth when he fought with the hero Walter of Aquitaine, an event related in the heroic epic Waltharius.
Hagen is never actually described as one-eyed in Nibelungenlied. It is true that Hagen loses an eye in Waltharius, but the author of Nibelungenlied did obviously not consider WalthariusCanon, because Gunther loses a leg in Waltharius and there is no indication that Gunther lacks one in Nibelungenlied. Hagen's one-eyed-ness is merely a tradition of illustrations and movies.
Removed this:
Hagen is never actually described as one-eyed in Nibelungenlied. It is true that Hagen loses an eye in Waltharius, but the author of Nibelungenlied did obviously not consider Waltharius Canon, because Gunther loses a leg in Waltharius and there is no indication that Gunther lacks one in Nibelungenlied. Hagen's one-eyed-ness is merely a tradition of illustrations and movies.
Let's just say and leave it at that.