Lancel is given very strong wine by Queen Cersei that is several times stronger than Robert is used to and told to give him as much as possible when Robert is hunting in the Kingswood. King Robert is killed by a wild boar.
After being knighted, Cersei takes him as her lover. When Tyrion Lannister finds out, he uses it as leverage to make Lancel his informer.
Lancel is badly wounded in the Battle of Blackwater and has a religious conversion from his near death experience.
Lancel is married and made a Lord, but he decides to give it all up in order to follow a religious life.