Complex sentence: To keep Nimue from causing damage, Merlin created the Dark One's Dagger via a tethering spell to control her; however, she stole it and used it to turn Merlin into a tree.
Split into simpler sentences: To keep Nimue from causing damage, Merlin created the Dark One's Dagger via a tethering spell to control her. However, she stole it and later used it to turn Merlin into a tree.