A fog-bound planet at the edge of the Badlands that served as a Maquis base and became the hiding place of the movement's last survivors, as seen in Blaze of Glory.
In Blaze of Glory, set in 2373, the Dominion has all but exterminated the Maquis after Cardassia joined it. Starfleet then intercepts a message suggesting that the Maquis have launched cloaked missiles at Cardassia, an attack that could provoke a war.
Sisko takes the imprisoned Michael Eddington from his cell and forces him to help. Eddington leads him by runabout to Athos IV, where the launch site is supposed to be, and the two fight their way past Jem'Hadar soldiers already on the planet.
There are no missiles. The message was a signal to Eddington that a handful of Maquis, among them his wife Rebecca Sullivan, were waiting on Athos IV to be rescued. Eddington is wounded and stays behind to hold off the Jem'Hadar, dying while Sisko gets the survivors away.