actually began his existence when Roland engaged in intercourse with a succubus
in order to have a vision that would aid in his quest for the Dark Tower. The gunslinger's sperm were carried in a form of stasis inside the succubus, which was in truth the androgynous demon, who would later take the form of an incubus
, along with the name Mia, and impregnate Susannah Dean. In the process, Mia's consciousness actually became intertwined with Susannah's (left porous by her preexisting personality disorder), and remained so until they physically met each other just before Mordred's birth.
Mordred was the literal son of Roland, but considered his true father, in a spiritual sense, to be the Crimson King. This was reflected in the dual nature of his birth: after being transferred from the body of Susannah into that of Mia, cell by cell, Mordred came into the world as a rather healthy (if unseemly) baby boy. When taken to Mia's breast to nurse, Mordred revealed his demonic nature by turning into a giant spider and draining all of her bodily fluids.
Mordred spent most of his life alternatively sleeping in his human form and feeding in his spider form. He grew and matured more rapidly when properly fed: after gorging himself on many small animals (and eventually larger ones), he would often have to sleep in his human form. His metabolism was much greater in his spider form, so he had to balance out how far he wanted to travel in his spider form with how plentiful food was in the area.
Mordred's most notable feeding came after his encounter with Randall Flagg, who intended to kill him and bring his foot to the entrance of the Dark Tower. Mordred sensed his intent, and displayed his telepathic abilities by assuming control of Flagg's body, a feat which Flagg himself had been known for in The Stand
. After this display of power, Mordred made Flagg tear out his own tongue and eyes and feed them to the spider. He ended the life of the Dark Man immediately thereafter.
Mordred spent the remainder of his life, the exact length of which is difficult to gauge, hunting and tracking down Roland. His motivation for wanting to kill Roland is never made entirely clear, but it's probably the strongest stated goal that Mordred has. During his pursuit Mordred undergoes malnourishment, extreme cold, and something like dementia: near the end of his travels, Mordred's control over his own powers begin to unravel, and his spider and human forms become less distinguishable.
He meets his end when attempting to kill Roland: except for Oy the billy-bumbler he most likely would have succeeded, and he manages to kill the animal, but Oy's barking and the sound of the fight alerts Roland to Mordred's presence. Before Mordred can attack Roland, the gunslinger puts four rounds into his head, killing him utterly, and leaves his corpse on the ruins of a campfire.