Cauldron of Arianrhod

"The Queen must enter the Cauldron. Its waters hold the Goddess's power."

- The Dochraid to Merlin

The Cauldron of Arianrhod is a lake located at the westernmost peak of an unspecified mountain range. It is a sacred site to followers of the Old Religion, as its waters hold the power of the White Goddess herself.

History
Merlin, Arthur, and Mordred took Gwen to the Cauldron of Arianrhod after it was discovered that Morgana had used a ritual called the Teine Diaga to bind her will to her own. According to the Dochraid, only the Cauldron's waters had the power to heal Gwen, but she must enter them willingly or her soul would be lost forever. Once she had entered the water, a sorcerer must then summon the White Goddess to break the enchantment.

To keep his magic hidden from Arthur, when they reached the Cauldron, Merlin disguised himself as an elderly sorceress called the Dolma and covered up his absence by telling Arthur (as the Dolma) that she was holding Merlin hostage to ensure her safety.

After making sure that Arthur understood the risks of what they were about to do, the Dolma woke Gwen from her drug-induced slumber and Arthur coaxed her into entering the Cauldron. Once Gwen was in the water, the Dolma summoned the White Goddess and Gwen was enveloped in white light, breaking the enchantment and restoring Gwen's true self (With All My Heart).

Trivia

 * Only sorcerers of great power are capable of summoning the White Goddess (With All My Heart).
 * Like many sacred sites, the path to the Cauldron is lined with banners that mark the way for pilgrims (With All My Heart).