Star Trek is no stranger to dark storylines or moments, every single show has at least one that sticks with them for the rest of their lives. From Picard being tortured and assimilated into the Borg, to Kirk being forced to let the woman he loves die in order to protect history, to plenty of others I'm probably forgetting.
And then you have Deep Space Nine, which can best be described as a series of super dark moments interspersed with Quark shenanigans.
Due to the lack of Roddenberry looking over their shoulder every step of the way for the first time, Deep Space Nine was allowed to broach subjects never allowed before in Star Trek. Not only making it easily the best entry in the franchise so far, but also making it by far the darkest.
This show is unflinchingly willing to hurt and even outright break the characters it made you fall in love with, and we're going to be looking at the darkest, bleakest examples thereof. These are the moments that made Deep Space Nine so freaking hardcore.