A Sidhe elder implanted a Sidhe into the infant Princess Elena, creating a Changeling. The faerie would have possessed Elena in 20 years' time, turning her into a Sidhe, when she would have married Arthur Pendragon (The Changeling).
When the Sidhe named Aulfric killed another immortal being, he was exiled from Avalon by the Sidhe elder with his daughter Sophia in human bodies (mortal shells). To re-open gateway between Albion and Avalon they needed the soul of a mortal Prince to offer to the elders. They went to Camelot in the name of Tír-Mòr and enchanted Prince Arthur, and took him to the lake and attempted to sacrafice him, but their plan was thwarted by Merlin (The Gates of Avalon).
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When the Great Dragon told Merlin to put the powerful sword begotten into his breath in a place where no mortal man can find it, he went to the Lake of Avalon and threw it into its depths (Excalibur).
When Merlin's love, Freya, died from her wounds, the young warlock put her body in a boat and made it flow onto the lake's waters, then he cast a spell, setting the boat
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on fire. As she was dying in his arms, Freya swore to Merlin that she would repay him. When the druid girl died, she was made the Lady of the Lake (The Lady of the Lake). Later, when Albian was in great danger and all seemed lost, Freya's spirit apeared to him in a pool of water from the lake of Avalon. She then returned Excaliber to Merlin so he could defeat Morgana, and save Camelot.
When Lord Godwyn, and his daughter Elena, payed a special visit to Camelot. Uther had plans to have Arthur
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marry the girl. Little did anyone know Elena's maid, Grunhilda was a pixie, and she was really the servant to the Sidhe elder, the same one who exiled Aulfric and Sophia. They were really working together to turn Elena into a Sidhe so they could have one of their own, in the heart of Camelot. But their plan was thwarted by Merlin and Gaius, and Arthur got out of marrying Princess Elena (The Changeling).
The Fisher King hands Merlin the Vial
When Merlin met the Fisher King, he gave Merlin water from the Lake of Avalon, so it will save Albion, in it's dark hour (The Eye of the Phoenix).
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When Morgause made Cenred's army immortal they became the living dead, then Morgause took charge and conquered Camelot and Morgana became queen. Merlin used the water from the Lake of Avalon that the Fisher King gave to him to contact his former lover, Freya. She told him to meet her at the lake, where she will hand him the sword he threw in earlier, with that he has the power to save Albion (The Coming of Arthur).
When Lancelot was forced by Morgana to kill himself, King Arthur ordered Merlin to give the knight the proper burial. Merlin took him to the Lake of Avalon, where he used magic to free his tormented soul. Lancelot was briefly revived, but he soon died. Then Merlin pushed the boat in which had placed the man's body in the middle of the lake and cast a fire spell to burn it (Lancelot du Lac)