A castellan (and/or a villainous knight) who engaged his younger brother, Sir Ontzlake (Hontzlake), in a land dispute, but avoided personal combat because he lacked confidence in his own fighting skill. He asked every knight who passed his castle to fight as his champion, but none would do so, so he threw all who refused into prison.
He eventually made an alliance with Morgan Le Fay, who transported Arthur to Damasís dungeon. Damas secured Arthurís promise to champion him against Ontzlake. Meanwhile, Morgan arranged for her lover Sir Accalon of Gaul (Accolon), with Excalibur, to fight at Ontzlakeís champion. With the assistance of Ninniane or Nimue, Arthur defeated Accalon and exposed Morganís scheme.
The Vulgate Lancelot says that Damas was reconciled with his brother, but in Maloryís version Arthur orders Damas to yield all his lands to Ontzlake, and to repay all the imprisoned knights. Arthur commanded that Ontzlake should hold the manor from Damas for the yearly fee of a palfrey, "for that will become [Sir Damas] better to ride on than upon a courser" - Damas had no business riding into battle on a warhorse, like an honorable knight.
A knight named Damas was killed during Lancelot's rescue of Guenevere from the stake. It might have been a different Damas, or it might be that Ontzlake's brother reformed and joined Arthur's court. Malory also mentions at least one and possibly two knights names Darras - and old knight with five sons who gives Tristram lodging after the tournament at the Castle of Maidens, and a knight of the Round Table who appears at the attempt to heal Sir Urre - and the names Damas and Darras may have been easy to confuse.
From the magical aspects of the adventure, it could be anywhere within the radius.
Damas of the Desert
Damas hated Lancelot's family because of their fame and skill. With four knights of similar opinions, he attacked Galahad and Bleoberis during the Grail Quest, and was slain by Bleoberis. He might be the same as the Damas above.