According to the Seer Taliesin, the Crystal Cave is the place where magic began, though whether he meant that the Cave itself was the source of magical energies or merely the place where the Old Religion was founded is unknown.
The First Visit Edit
Merlin first visited the Crystal Cave when he and Arthur were ambushed by bandits in the Valley of the Fallen Kings. They managed to escape, but Arthur was badly injured and Merlin was unable to heal him with his magic.
As the prince appeared to be nearing death, Taliesin appeared and used magic to heal him. He then took Merlin to the Crystal Cave and urged him to look into the crystals, assuring him that much would be revealed if he did.
Merlin initially refused, as the last time he'd looked into the future he'd seen terrible things, but Taliesin coaxed him into looking anyway by suggesting that there was a reason that he'd been brought there at that moment in time. When the warlock asked what that reason was, Taliesin replied that only the crystals could tell him, and urged him once more to look into them.
Merlin reluctantly did as the Seer asked and focused his attention on the crystals. He saw a number of visions in their depths, including one of Morgana murdering Uther. He later attempted to avert this future, and though he succeeded, he did so with disastrous consequences; he actually wound up causing most of the things he'd seen (The Crystal Cave).
Magic Itself Edit
After he was stripped of his magic by the Gean Canach, Merlin traveled to the Crystal Cave in the hopes of regaining his powers. Soon after he entered the Cave he found Morgana waiting for him, and the High Priestess used her magic to create a rockslide that trapped him inside. Injured and heartbroken at having failed in his mission, the warlock stumbled through the Cave until he reached a cavern full of crystals, where he collapsed.
Merlin was awoken some time later by a soul-like apparition of his father Balinor. The specter urged him not to give up and told him to let hope into his heart. When the warlock asked what hope there was without his magic, Balinor replied, "You are more than the son of your father. You are son of the earth, the sea, the sky. Magic is the fabric of this world, and you were born of that magic. You are magic itself. You cannot lose what you are." He then told Merlin to rest, and to believe in what his heart knew to be true, "And soon you shall awaken into the light."
When Merlin woke up, his injuries had been healed and his magic restored. He tested the latter by incanting a spell to create a butterfly, then used his newfound command over the crystals to observe Arthur and telepathically warn him about Morgana's plans. His task complete, he thanked his father for his guidance and told him that he followed in his footsteps, to which Balinor replied that his journey had only just begun. He told Merlin that he wielded a power that he couldn't yet conceive of, and that only in the heart of the Crystal Cave would his true self be revealed.
Balinor then pointed Merlin towards a light at the center of the Cave and urged him to move towards it. "Your destiny awaits. Do not be afraid. Trust in what you are, trust in what you will be." Taking his father's advice, Merlin bid Balinor goodbye and followed the light deeper into the Cave.
The Crystals Edit
It is unknown whether all sorcerers are capable of seeing visions in the crystals or only certain sorcerers. However, according to Kilgharrah, the Crystal of Neahtid (which was hewn from Crystal Cave) could only be wielded by powerful sorcerers, which may imply that only highly skilled sorcerers had the power to command the crystals (The Witch's Quickening, The Crystal Cave).
- The Crystal Cave was said to be the source of Taliesin's power of prophecy (3x05).
- According to Gaius, in the time before the Great Purge wizards would speak of the Cave with reverence (3x05).