In 1387, his brother died under Aragonese captivity and Marianus became sole judge at a time when the Giudicato of Arborea was at war with the Crown of Aragon.
Pada tahun 1387, abangnya yang dipenjara meninggal dan Marianus menjadi Giudice tunggal pada saat Giudicato Arborea berperang dengan Mahkota Aragon.