Equilibrium

Jadzia suddenly plays an unfamiliar piece of music, flies into uncharacteristic rages and is haunted by visions of a masked figure. Bashir finds that her bond with the Dax symbiont is breaking down, and she is taken to the Trill homeworld for treatment. The Symbiosis Commission is unhelpful, but the music leads them to Joran Belar, a composer whom official records say was never joined. They discover that Joran was in fact a host of Dax, and that he had committed murder. The Commission had erased him from history and suppressed his memories in the symbiont, because the case exposed a secret: far more Trill, around half, are capable of being joined than the public is told. To save Jadzia's life, she is allowed to confront and accept Joran's memories, and she recovers as they become part of who Dax is.