in the romance of The Round Table, is represented as the treacherous knight. He revolted from his Uncle Arthur, whose wife he seduced, was mortally wounded in the battle of Camlan, in Cornwall, and was buried in the island of Avalon.
The nephew of King Arthur. He hated Sir Lancelot, sowed discord amongst the Knights of the Round Table, and tampered with the “lords of the White Horse,” the brood that Hengist left. When the king went to chastise Sir Lancelot for tampering with the queen, he left Sir Modred in charge of the kingdom.
Modred raised a revolt, and the king was slain in his attempt to quash it. (Tennyson: Idylls of the King; Guinevere.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894