Le Morte d'Arthur, written by Sir Thomas Malory, was published by William Caxton as 21 books. Sir Lancelot (spelt "Launcealot by Malory) appears first in Book II, when Merlin prophesies that " No man shall handle this sword except the best knight in the world, and that will be Sir Launcelot or else Galahad his son, and with it Launcelot shall slay the man he loved best in the world, and that will be Sir Gawain."
Malory distances Lancelot from much of the passion between the two lovers seen in the Vulgate Cycle and concentrates on Lancelot success, rather than his eventual failure.
Lancelot gradually moves centre stage in Malory's works - 'The Noble Tale of Sir Launcelot du Lake' (Book VI) in which he declares his love for Guinevere (Malory uses "Gwenyvere"). Lancelot knights Gareth (Book VII - 'The Tale of Sir Gareth of Orkney'). Lancelot rescues Gawain.In 'The Tale of Sir Tristram de Liones' (Book VIII). He rescues La Cote Male Taile (Book IX). Lancelot is tricked into sleeping with Elaine (de Corbenic), thinking her Guinevere, and hence Galahad is conceived (Book XI). Lancelot becomes mad and a knight, Bliant, takes Lancelot to his castle to cure him of the madness (Book XII). He is persuaded by Ector to return to Arthur's court, where Lancelot knights his son Galahad (Book XIII).
The knights go on a quest of the Holy Grail. He has a vision in which he sides with the black (sinful) knights against the white (pure) knights (Book XV). Lancelot returns to his love of Guinevere (Book XVIII) and arrives at a tournament to defend her, wearing the sleeve of Elaine of Astolat. He is wounded and Elaine dies for love of him.
Meliagaunt (Méléagant) abducts Guinevere (Book XIX), but he and Guinevere are discovered in bed together (Book XX). Lancelot escapes, but Guinevere is captured and condemned to death by burning at the stake, for her adultery. Lancelot returns to rescue the queen from the stake. Gawain makes war on Lancelot who slays Gareth.
Finally (Book XI) Lancelot and Guinevere part and he leaves Camelot to become a monk.