Mordred Deschain was a were-spider who was the half-son of Roland Deschain (White Father) and the half-son of the Crimson King (Red Father) born through the succubus Mia while she inhabited the body of Susannah Dean. Mordred was born inside the Fedic Dogan and immediatly changed to his Were-spider form and killed Mia by sucking her dry. Susannah watched in horror as this happened and pulled Richard P. Sayre's gun from its docker clutch and shot at Mordred, ripping one of his spider legs off. For the remainder of the series, he is missing this leg and his human form has a scar on his side, where the leg would otherwise be.
Mordred was long prophisized in Mid-world. He was said to be born of two fathers through incest with his daughter/sister. Roland's sperm was taken from him by an elemental demon in the oracle during The Gunslinger. It was taken and mixed with the sperm of the Crimson King. While "drawing" Jake Chambers to Mid-World, Susannah has sex with the hermaphrodidic demon Roland had sex with earlier and is impregnated with the mixed sperm.
Mordred murdered Walter Padick after telepathically detecting Padick's intention of using Mordred's birthmark to enter the Dark Tower. Mordred follows the Ka-tet of Roland for a long time, having multiple opportunities to attack. Finally when it is just Roland, the artist Patrick Danville, and the billy-bumbler Oy, Mordered attacks. Oy sacrifices his life for Roland, allowing the gunslinger to shoot and kill Mordred close to the fields of Can-Ka-no-rey. He is named after the betrayer of Arthur.