When Star Trek: Deep Space Nine aired in 1993, it set itself apart from any Star Trek show preceding it with its primary location of a large space station, darker tone and a later-season focus on serialized storytelling. The series thrust its characters into conflict with the Dominion, a military force comprised of many species from the Gamma Quadrant who invaded and threatened Starfleet and the Alpha Quadrant like no other enemy before.
Source：https://www.cbr.com/star-trek-why-terry-farrell-jadzia-dax-left-ds9/
