Crystal Cave

''"This is the Crystal Cave. This is where Magic began. Use what you see for good."

- Taliesin to Merlin The Crystal Cave is a large cave filled with white crystals located in or near the Valley of the Fallen Kings. According to the seer, Taliesin, it is the place where magic began, though it is unclear whether he meant that the cave was the source of Magical energies or that the Old Religion was founded within it. In the time before the Great Purge, Wizards talked about the cave with reverence.''

The crystals within the cave had the power to show those who looked into them potential futures, Taliesin credited his powers of prophecy to the cave, and it is the place where the Crystal of Neahtid was mined from.

History
After Arthur and Merlin were chased into the Valley of the Fallen Kings, Taliesin showed Merlin  the cave, where he saw visions of Lady Morgana killing Uther Pendragon with an ornate dagger. He attempted to avert this future but did so with disastrous consequences, and ended with him in fact causing it. However, he forgot about the first prophecy that he saw, which depicted Morgana as queen, or simply thought of it as a possible future, since Gaius said "the Crystals are treacherous" (The Crystal Cave).