Despite running for seven seasons and telling dynamic stories with myriad tones, Star Trek: Deep Space Nine earned the reputation as the "dark" show in the franchise. While this reputation is a bit overblown, one of the show's most important episodes likely helped that idea stick. Season 6's "In the Pale Moonlight," is a daring DS9 story that pushes Captain Benjamin Sisko, and the audience, into very uncomfortable territory.
Source：https://www.cbr.com/ds9-s6-e19-important-episode/
