In 2371, O'Brien's mirror counterpart Smiley, sneaks aboard Deep Space 9 to steal Defiant schematics. While there, he also acquires schematics for the Phoenix-X. Four and a half years later, in 2375, the mirror Phoenix-X is created, but quickly destroyed. The only person to survive is the Executive Commander of that vessel, James, who returns six and a half years later (a.k.a. the present, a.k.a. 2382) to capture the Phoenix-X in the original universe. With the crew hand-cuffed and taken prisoner aboard their own ship, James decides to bring the Phoenix-X into the Mirror Universe, where he intends on selling the crew as slaves for a position within the Klingon-Cardassian Alliance. Unfortunately, due to a quantum chemical imbalance within Seifer's symbiont, experienced during the Phoenix-X's encounter with the Narendra Temporal Rift, Seifer is unable to contain transformations and sentience brought about within his symbiont. It isn't long before his symbiont escapes him and begins, yet again, running a muck, and taking James' plan for itself.
Seifer's symbiont being genetically enhanced was done in the episode "Occurrence, Part I", where the symbiont was influenced by the Quantum state as well and allowed to experience sentience and separation on its own.