The Dorocha's Attack

"What you are about to do will affect everyone. But most importantly, it will bring our enemies to their knees."

- Morgause to Morgana about the Dorocha's release

This event happened when Morgana, who still wanted revenge on Camelot, performed 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, the spirits of the dead, known as the Dorocha, emerged and attacked Camelot after destroying Drea's 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 jumped out in front of a Dorocha which was about to attack himself and Arthur, to save Arthur, and became gravely ill. Merlin was later healed by the 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 that could not happen, as his time with men was not yet over, even if he wanted it to be. 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).