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Blackwell traps Cassie and Diana in a circle spelled with dark magic so they can't escape. He reveals his real plans is not to help kill the witch hunters, it's to build a circle filled with dark magic witches. Cassie and Diana try to escape but the spell is too strong for Cassie's dark magic alone so she forces Diana to activate her dark magic so they can break the seal on the spell and take down Blackwell.
Source: The Secret Circle - Season 01, Episode 22 - "Family"
NO COPYRIGHT INFRINGEMENT INTENDED