Before the Crystal Alicorn blew their amulets into sparkly shards, the Sirens received a painful shot to the heart during the Battle of the Bands.
They're out of energy, out of immortality, and out of options.
Fully human now, the Sirens have no access to magic to heal the damage caused by the Rainbooms' spells. But Rarity, Fluttershy, and Sunset Shimmer aren't about to ignore their plight.