With the Rainbooms down for the count, Sunset Shimmer steps in to help them out and the students begin to sing along with them. The Six overwhelm Aria and Sonata at first, but Adagio's desperate powerful sonic waves throw the girls off-balance, allowing the Dazzlings to gain the upper hand.

In turn, the Sirens summon astral projections of their true forms for the battle. Just as they are about to devour the combined energy of the students, the Rainbooms appear to stop them. However, thanks to Spike, the Rainbooms managed to escape. The Dazzlings then trick Trixie into removing the Rainbooms from the competition altogether by trapping them in a locked basement. This causes resentment in their opponent Trixie, who refuses to accept this. Feeling that simply getting the Rainbooms eliminated might not do the trick, the Sirens arrange that they pass to the finals instead despite a lackluster performance. When they see Sunset Shimmer again, they threateningly play with her emotions and intimidate her, claiming that 'no one is going to remember her when they win'. In the meantime, they continue gathering energy from the other students. This turns the Rainbooms into priority targets for the Dazzlings. The sirens realize that this group of students were not under their control and deduce that they must have great magic in them - and thus, great power to absorb, though they would have to turn them against each other without the use of their own magic first.

Instead, their failed attempt plays right into the Dazzlings' hands. Twilight discovers the true nature of the Dazzlings, but when she and the rest of the human Six attempt to defeat them, they realize that they are not able to unleash the Elements of Harmony at will. They also manipulate Principals Celestia and Luna into bending to their will. In the cafeteria, they sing a song that ignites the students' competitive spirits, turning a planned friendly singing festival into a Battle of the Bands. The arrive in Canterlot High and Sunset Shimmer gives them a tour around the school, but she feels a bad vibe from them and notices that Adagio stops her from touching her crystal necklace. Now once again in possession of their magical abilities, they decide to take over the Human World, starting with Canterlot High, the source of the magic. They realize that magic has been brought into the Human World (due to Sunset Shimmer's theft of the Element of Magic), and that their own power has returned as a result. Then they witness the Harmony blast that defeated Sunset Shimmer in the previous movie. The three sirens are first seen in a city cafe, where they are observed absorbing negative energy from the customers there, complaining about the miserly amounts of power they gain from this. Unable to defeat them, the great mage Starswirl the Bearded resorted to banishing them to another dimension, where magic was nonexistent and their power was nullified, or at least reduced to nothing but a pale shadow of what it once had been.

The three Sirens were once powerful threats to Equestria itself, sowing chaos with their enchanted voices and growing ever more powerful from feasting on negative emotions. The Sirens in a flashback from Shadow Play Part Two.

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