When his brother-in-law, the emperor Henry, died without issue in 1216, Peter was chosen as his successor, and with a small army he left his residence of château de Druyes in France to take possession of his throne.

He was consecrated emperor at the Basilica of Saint Lawrence outside the Walls in Rome by Pope Honorius III on 9 April 1217.

He then borrowed some ships from the Venetians, promising in return to conquer Durazzo for them, but he failed in this enterprise and sought to make his way to Constantinople by land.

[4] On the journey he was seized by the despot of Epirus, Theodore Komnenos Doukas, and, after an imprisonment of two years, died,[4] probably by foul means.

Peter thus never governed his empire, which, however, was ruled for a time by his wife, Yolanda, who had succeeded in reaching Constantinople.