HistoryEditAfter discovering Morgana's plot against Arthur's life, Merlin and his friend Gwaine rode after the prince and joined him on his quest to recover the Fisher King's trident. As the trio searched the King's castle, Merlin became separated from his friends and found himself trapped in the throne room. There he met the Fisher King himself, who had awaited his arrival for hundreds of years.
The Fisher King revealed that the quest was not Arthur's but Merlin's, and the true prize was not his trident but a glass vial containing water from the Lake of Avalon, which he had guarded during his long wait. He warned Merlin that Albion's time of need was near and that he alone could save her, but that he would need help, and the water would provide that help (The Eye of the Phoenix).
When Morgause and Morgana conquered Camelot with their immortal army, Merlin took the vial with him when he, Arthur, and their allies went into hiding. He experimented with the water using magic and various spells, but to no avail. Whatever help the water could provide remained a mystery, and he eventually dozed off with the vial still in hand.
Sometime later, Merlin was startled awake by the sound of Gwaine getting up in the night, and in his surprise the vial slipped from his fingers and shattered on the cave floor. Horrified, Merlin watched helplessly as the water began to trickle away. Much to his surprise, however, breaking the glass had freed Albion's last hope rather than destroyed it. The water took on an ethereal glow and formed a small puddle in a crevice of rock, through which he was able to contact his deceased lover Freya.
Freya explained that the Cup of Life had not only made Morgana's army immortal but transformed them into the living dead, and that there was only one thing that could slay that which was already dead: the sword Excalibur, which Merlin had hidden at the bottom of the Lake of Avalon. She promised to give him the sword when he came to the lake, and thus provided him with the help he and Arthur needed to retake Camelot (The Coming of Arthur).