Complex sentence: Merlin proved too powerful, however, and they were all banished into the Wild Magic-once here, she was unable to return to Avalon without her Dark Stone, drifting as part of the magic itself.
Split into simpler sentences: Merlin proved too powerful, however, and they were all banished into the Wild Magic. Once here, she was unable to return to Avalon without her Dark Stone, drifting as part of the magic itself.