Merlin fell in love with Nimue when King Arthur retrieved Excalibur from the lake. Nimue's incentive to preserve their romance was to gain the knowledge of magic that the wizard had.
She learned a spell from him that could entrap a person for all time. After Lancelot rescues Guinevere and before the battle at Joyous Gard, Nimue casts the spell on Merlin. The sources differ in what Merlin was trapped in. Some say that it was in a Crystal Cave, while others say that he was trapped in a Castle of Air. This spell was irreversible.