Lancelot du Lac was Darthene's
lover when she was Queen of Avalon, and
Andelia's father. He was Darthene's strong right hand, found in
company with her at almost all times, her knight, her friend, her
confidant. He was Andelia's foster-father as she grew up, but while
the relationship was not made clear during his life, she treated him as
her father and he treated her as his daughter. It was he who first
taught her the way of the sword and of the knight, teachings that have
stuck with her ever since. As far as is known, he died during the
climactic battles of the war over Avalon between Darthene and Corwin,
though only Corwin could say for certain (he was alive when last
Darthene or Andelia saw him).
The events of NightSummer have shed more light on Lancelot. He is alive still, because he is the son of Morgan, though at that time Morgan was going by the name of Mordred. Mordred and his mother Darthene had been on bad terms for some time, so Mordred did not inform her that she had a grandson. When Mordred disappeared on the Rebman Pattern, and thus from Avalon, the young Lancelot assumed that Corwin, Mordred's mentor and rival-to-be, had killed his father. So, he joined Corwin's enemy, Darthene, and became her knight, her friend, her lover, the father of their daughter Andelia. He escaped Avalon before it's final destruction, and wandered Shadow for centuries, looking for the clues to the Rhyme of Opal Fire, that would allow him to, at Nightsummer, resurrect his `dead' father.
On finding his father and his daughter alive at the Lighthouse of Cabra just prior to Nightsummer, Lancelot's quest was fulfilled, though in a different way than he'd expected. His world crashed soon afterwards, though, when he learned from Morgan that Darthene was Morgan's mother, and thus his grandmother. The woman that he'd loved, the mother of his daughter, was his grandmother. His heart, strained to the breaking point by the stress of the quest he'd been on, gave out, and he fell unconscious. Raven took him to Kashfa, where he recovered from his heart attack.
Distraught and shaken by this revelation, he retreated to a
monastery and took a vow of silence. His daughter Andelia wasn't about
to accept this, though, and continues to visit him, to the point where
he has now renounced silence. Always a quester, he is now beginning a
quest for his lost lover, his grandmother Darthene.