The pagan Lord of Derudicanoro in the Dark Valley.
Tristan and Lancelot came to his fortress during the Grail Quest. Both fought with him and were overwhelmed by his strength. Lancelot was imprisoned while Tristan was left for dead.
Upon regaining consciousness, Tristan pursued Fellone into castle Derudicanoro and was able to defeat him by swearing before God that he would not sin again with Isolde. Fellone agreed to be baptized and to swear allegiance to Arthur.
Source
La Tavola Ritonda | 1325–1350