The eyepiece headset worn by the commanding officers of Jem'Hadar warships in place of a main viewscreen, which gives humans headaches when Sisko's crew flies a captured vessel.
Dominion warships have no viewscreen and no chairs. Instead the Jem'Hadar First and the supervising Vorta each wear a small headset with a display over one eye that shows the view outside the ship directly to the wearer.
The device features when Starfleet operates the Jem'Hadar attack ship captured in The Ship. In A Time to Stand, Sisko and Garak wear the two headsets on the mission to destroy a ketracel-white facility. Sisko suffers painful headaches from it, while Garak, as a Cardassian, copes rather better.
The headsets are seen again in Rocks and Shoals and on Dominion bridges throughout the war, and help to make the interior of Jem'Hadar ships feel markedly alien and austere.