Garlan
Brother of King Pellehan (Pellam) of Listenois (a Grail King) who appears in the Post-Vulgate Merlin continuation, Malory, and Tennyson. He loathed Arthur and the Knights of the Round Table.
With the power to turn himself invisible, he enjoyed killing other knights by thrusting lances through their backs. Two of his victims were Sir Harlews and Sir Peryn. The populace thought there was a fiend on the loose. Arthur’s Sir Balin the Savage, investigating these killings, tracked Garlon to King Pellehan’s castle, where he joined a feast in Pellehan’s hall. When Garlon showed up visible at his brother’s banquet in Listeneise, he is described as having a “black face”, which here probably means dour.
Balin was hesitant to confront Garlon in the castle, but he became enraged when Garlon either slapped him in the back of the head (Post-Vulgate and Malory) or insulted Queen Guenevere (Tennyson). Balin stood up and clove Garlon through the head. Pellehan, enraged, insisted on fighting Balin, chasing Balin through the castle until Balin found Longinus’ Spear and struck Pellam in self-defense. Thus Garlon became the immediate occasion of the Dolorous Stroke.
Tennyson says that Garlon was a lover of Vivien.
Sources
Post-Vulgate Suite du Merlin | 1230-1240
Le Morte Darthur | Sir Thomas Malory, 1469-1470
Idylls of the King | Lord Alfred Tennyson, 1859-1886