A mute madman at Arthur’s court.
When Gaheris and Agravain first came to court, Marins suddenly recovered the power of speech, told Arthur to knight Gaheris before Agravain, prophesied greatness and tragedy for Gaheris (he had received this prophecy from Merlin), and then died.
Arthur had him buried at the Church of St. Stephen’s in Camelot.
Source
Post-Vulgate Suite du Merlin | 1230-1240