Jadzia was played by noted vampire Terry Farrell. Like all members of the undead, Ms. Farrell could not be in contact with direct sunlight and preyed upon the living for sustenance. Many grips and interns were killed over the first six seasons of DS9 to keep the soulless actress nourished.
She left the series once Paramount was acquired by a division of Van Helsing Enterprises.
In the much cooler MAlf timeline, she didn't die, like in the ridiculous MAlph timeline. Instead, when Sisko ascends to the Admiralty, she is made permanent CEO of the USS Too Defiant to Die. She even wears a proper red turtleneck, only when on duty, of course.
The Defiant was too OP so was not destroyed by Neil Breen's Energy Soaking Weapon. Dax was eventually given to a slightly bigger ship to command called the USS Atlantis (in name only), which was Intrepid Class of '75, kinda like Voyager, except with a much sexier captain.