12 Darkest Star Trek: Deep Space Nine Moments
11. "We're Everywhere" - The Adversary
Plenty of shows like to end certain seasons on dark cliffhangers, but few of them get as harrowing as the last moments of The Adversary.
The 3rd season is where we get our first glimpses of The Dominion, and the season finale is where the crew of DS9 figures out just what they're up against. Throughout the episode, a changeling infiltrator almost tears the entire ship apart by turning the crew against each other, demonstrating how easy it is for just ONE changeling to infiltrate and gum up the works of the federation by making officers kill each other.
Odo manages to bring down the changeling, but the moment that makes it onto this entry is the last message of the changeling to his traitorous brother: we're everywhere.
Those last words being the last moments of the season leave viewers with a haunting realization. The Dominion is already making their move, they know everything about the federation and how to wreak havoc within their ranks, and it may already be too late to stop them.