“The Jem’Hadar” is episode twenty-six of season two of Star Trek: Deep Space Nine and the season finale.
Sisko and his son are going on a trip to the Gamma Quadrant when Jake invites Nog to join them too. When Quark hears about this, he joins them as Nog’s guardian – although it’s more to do with a chance to talk to Sisko about his latest scheme.
On the planet they visit, Jake and Nog are away from the camp when a woman rushes in, quickly followed by soldiers who take Sisko and Quark prisoner. The soldiers are the Jem’Hadar, the most feared warriors of the Dominion – the latter being an interstellar polity in the Gamma Quadrant which has cropped up in mentions a few times.
A Jem’Hadar ship visits DS9 and warns them off visiting the Gamma Quadrant. The Jem’Hadar visiting the station gives Major Kira a list of all the ships they have destroyed, and also evidence that they’ve destroyed the colony of New Bajor too.
The Dominion seems to be a serious threat, as they have technology that is getting past the Federation’s shields.