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Something dark is brewing over in Storybrooke in the season 4 finale of "Once Upon a Time." 

The two-part finale titled "Operation Mongoose" is slated to premiere on Sunday, and it is expected to revolve around the friendship of Emma Swan (Jennifer Morrison) and Lily (Agnes Bruckner), as well as the plans of the Author regarding the happy ending of the major characters of the series. 

The second half of the season began with the introduction of three evil queens, namely Maleficent (Kristin Bauer van Straten), Cruella de Vil (Victoria Smurfit), and Ursula (Merrin Dungey). All of them hounded Storybrooke to find the Author so that they could get their happy endings. 

Ursula got her happy ending when her father King Poseidon returned her singing voice, while Cruella died in the hands of Emma to save her parents. Only Maleficent is left to find out if she'll have her happy ending with her daughter Lily or if she is also doomed to fail. 

Maleficent actually managed to spend time with her long lost daughter in the penultimate episode of "Once Upon a Time" season 4, but their happy reunion may not last for long since Lily is determined to get her revenge on those responsible for her growing up away from her mother. However, the people who she plans to take her revenge on happen to be the parents of her childhood friend Emma. Will Lily succeed or will Emma stop her from hurting Snow White (Ginnifer Goodwin) and Prince Charming (Josh Dallas)? 

Meanwhile, the second part of the season finale will deal with the Author's plans to rewrite the story based on the demands of Mr. Gold (Robert Carlyle), the Evil Queen Regina (Lana Parrilla), and the Wicked Witch (Rebecca Mader).

The two-part finale special of "Once Upon a Time" season 4 will air on Sunday, May 10.