A great many sorcerers died defending the Crystal during the Great Purge. Though Uther had no idea of its purpose, the fact that they'd gone to such lengths to protect it led him to believe that the Crystal was an important instrument of magic and he had it locked away in the castle vaults.
Roughly twenty years later, sorcerers Mordred and Alvarr plotted to use the Crystal to kill Uther and destroy Camelot. They recruited Morgana to their cause and persuaded her to steal it from the castle vaults. She brought it to their hideout the next day, but their camp was discovered by the Knights of Camelot soon afterward and the Crystal was reclaimed by Arthur.
That night, Arthur ordered Merlin to guard the Crystal while he and the other Knights rested. The warlock was oddly transfixed by the Crystal and refused even to hold it at first, but in the end he was unable to resist its pull. He snatched it up from the ground and held it, and when he looked into its depths he was horrified to see a vision of the Great Dragon attacking Camelot.
The Crystal was presumably returned to the castle vaults after the Knights returned to Camelot (The Witch's Quickening).
Ability EditLike the other crystals of Crystal Cave, the Crystal of Neahtid contained knowledge of the past, present, and future. Some sorcerers even believed that it held the secret of time itself.
According to the Great Dragon, the Crystal could only be wielded by powerful sorcerers such as Merlin or Mordred. He claimed that Morgana did not have the power to wield the Crystal, though whether he meant that her magic currently wasn't strong enough or that she didn't have the ability to is unknown (The Witch's Quickening).