A rare mutated Ussal, many times the size of a normal one, resided in Metru Nui. It possessed inferior sight and developed greater hearing to hunt. It was kept in a sub-level of the Onu-Metru Archives, as it was resistant to the effects of stasis.
1,001 years ago, while searching for the Onu-Metru Great Disk, Toa Metru Nuju and Whenua looked within the beast's cell, provoking it. The Ussal attacked the two Toa viciously. Whenua generated a deafening hum from his Earthshock Drills, using the creature's sensitive hearing against it. The mutant Ussal hurled the two Toa out of its cell, and they locked the door upon it. After the Great Spirit Robot was critically damaged, it is unlikely that the Mutated Ussal survived as it was trapped in its cell.