Sadok was one of Mark's men, and when Mark murdered Prince Boudwin, he sent Sadok to bring back Boudwin's escaping wife Anglides and infant son Alisander. Sadok caught up with Anglides within ten miles, but let her go on condition she raise her son to avenge his father's death (which she may have intended to do anyway). Sadok returned to Mark and told him he had faithfully drowned Boudwin's son.
Years later, hearing that Alisander had just been knighted, Mark realized that Sadok had betrayed his orders. Mark and some of his knights tried to kill Sadok at once, in the castle. Sadok fought and killed four knights in Mark's presence, then escaped, Tristram, Dinas, Fergus, and the other true-hearted knights about the place being in sympathy with him. Mark sent yet another "false knight" after Sadok, and Sadok slew this one, too. Then Mark sent messages to Morgan and the Queen of Norgales, enlisting their aid against Alisander - and perhaps also against Sadok.
Sadok must have either remained at large in Cornwall or secretly returned to the vicinity, for when Mark put Tristram in prison, La Beale Isoud appealed to Sadok. Sadok and two of his cousins ambushed Mark's party near Tintagil. Sadok lost one of his cousins but slew Mark's four nephews and at least one "traitor of Magouns [Castle]", then rode on the castles of Lyonesse and Arbray, where he joined Sir Dinas and they roused the country to rebellion.
Meanwhile, Percivale effected Tristram's release, and Mark tricked Dinas into disbanding the rebellion by pretending to be about to go on Crusade at the Pope's command. Mark put Tristram back in prison, Isoud appealed to Dinas and Sadok again, and this time Sadok presumably helped Dinas put Mark into prison long enough for Isoud to deliver and escape with Tristram. It is possible that Sadok and Dinas joined Tristram and Isoud at Joyous Garde.
Malory next mentions Sir Sadok and his cousin Sir Edward at the Lonazep tournament, where he calls them cousins of Sir Gawaine. (It is possible this is a different Sadok, but it is probably the same, for Gawaine's mother Margawse was of Cornish birth.)