"Eclipse" by Cate Tiernan is the twelfth book in the Sweep series. When Morgan's birth father sends a dark wave to her town because she refuses to join him in his evil magick, Morgan, Hunter, and Daniel try to find a way to counter the destructive force. They find their solution in Alisa who has just discovered that she is a half witch with significant powers. "Eclipse" is an engaging novel that emphasizes the good that can be accomplished by overcoming one's fears.

During Kithic's Saturday circle, Ciaran, Morgan's evil birth father, appears to warn Morgan that he wants to talk to her. When Morgan and Hunter, her boyfriend and the leader of Kithic, cry for Morgan, they see a vision of an approaching dark wave, a powerful force of destruction, being sent to Widow's Vale. The next day, Morgan acquires an odd green-covered Book of Shadows from Practical Magick. Though Alisa, a local girl who is friends with Morgan's younger sister Mary K, is afraid of Morgan's uncontrollable powers, she is drawn to the Book of Shadows and steals it from the Rowlandses' house. When Alisa learns that the Book of Shadows belonged to her mother, Sarah Curtis, she realizes that she is half-witch. Devastated at the idea, she turns to Morgan for support. Ciaran visits Morgan and asks her to join him, but she refuses so he tells her that there will be penalties, alluding to the dark wave.

Meanwhile, Daniel believes he can create a spell to counter the dark wave, but the witch who performs the spell will likely die from their proximity to the dark wave which is already causing all the witches in Widow's Vale to feel ill. Though Alisa finds peace at Kithic's circle, she notices that Hunter and Morgan seem extremely sick. Since Morgan will not tell Alisa what is going on, Alisa questions Hunter and learns about the oncoming dark wave and Daniel's attempt at a counter spell. She realizes that the dark wave does not affect her as strongly since she is only half witch so she offers to learn the spell, suggesting that she may be able to survive the dark wave if she casts the spell. Daniel and Hunter begin coaching her.

As Alisa practices the Nal Nithrac spell to defeat the dark wave, Morgan feels guilty that everyone is in danger because of her relationship with Ciaran, so she convinces Ciaran to meet her at the local power sink. Ciaran tries to persuade Morgan to join him, insisting that she is too power to stand in opposition to him, but when she refuses, Ciaran is furious and puts a binding spell on Morgan. Morgan breaks Ciaran's concentration, uses his true name to force him to release her and places a binding spell on Ciaran before calling Hunter and Daniel for help. When Hunter and Daniel arrives, Morgan explains what has happened, so Hunter calls three additional witches to help strip Ciaran of his powers while Morgan convinces her half-brother, Killian, to come to Widow's Vale to take responsibility for Ciaran's care after his powers are stripped. Morgan feels guilty for stripping Ciaran of his powers, but she is relieved that the dark wave is no longer a threat.

Several days later, Morgan and Hunter still feel sick. When Morgan notices that a plant that has recently died, they realize that Amyranth, Ciaran's coven, is still sending the dark wave to Widow's Vale, so they call Alisa. Though Alisa is not ready to perform the Nal Nithrac spell on her own, Hunter has an idea; Alisa performs the spell at the power sink while Morgan and Daniel maintain a mental connection to her from a distance, allowing Daniel to coach Alisa through Morgan. Although it is difficult, Alisa manages to cast the spell and defeat the dark wave. All of them are very happy that they defeated the dark wave. Alisa accepts her status as a half witch and plans to learn more about using her powers, while Morgan is simply happy that she is now able to look forward to the future now that Ciaran and the dark wave no longer present a threat.