The origin of this Pharaoh is very doubtful. One theory says that Amenmesse and Messui, the viceroy at the time of Merenptah, were the same person - the future usurper, who ruled wrongfully for more than 4 years at the time of Seti II. Another theory says that Amenmesse was a vizier of Merenptah.
The Amenmesse reign period is one of the most unclear in the history of Egypt. K. Kitchen believed that Amenmesse also ruled in the north. His final resting place is the tomb KV 10 in the Valley of the Kings and there are traces of devastation from the time of Seti II. (it is highly propable that he ordered the devastation), which indicated illegal rule of Amenmesse.