The Dorocha's Attack

This event happened when Morgana, who still wanted revenge on Camelot, did a blood sacrifice on Morgause on samhain's eve to tear open the veil between the living and spirit worlds. When the veil was torn, creatures known as the Dorocha emerged and attacked Camelot after destroying a village. Arthur, who took over Camelot as Uther was broken by Morgana's betrayal, learned from Gaius that the only way to defeat the Dorocha would be to make another sacrifice to heal the torn veil. Arthur went on a quest to the Isle of the Blessed along with Merlin and the Knights. Along the way, Merlin sacrificed himself to protect Arthur from a Dorocha and was gravely ill. Merlin was later healed by Vilia and rejoined the others. They eventually arrived at the Isle of the Blessed and encountered the Cailleach, the gatekeeper to the spirit world. Arthur planned to sacrifice himself to heal the veil but Merlin knocked him unconscious to confront the Cailleach. When attempting to sacrifice himself, the Caileach told him that it could not nhappen as his time with men was not yet over. Lancelot, who chose to sacrifice himself, then walked into the veil and vanished as the veil was healed, defeating the Dorocha. Arthur, Merlin and the other Knights returned to Camelot and mourned Lancelot's death. (The Darkest Hour)