Ultritium is an extremely powerful chemical explosive, virtually undetectable by conventional sensors and therefore frequently employed in covert operations. Ninety isotons of enriched ultritium had enough explosive power to vaporize almost all matter in a radius of 800 kilometers.
In 2365, a pair of Antedean assassins posing as a diplomatic delegation attempted to bomb a conference on Pacifica using ultritium. Their plot nearly succeeded, were it not for Ambassador Lwaxana Troi, who read their minds. (TNG: "Manhunt")
An ultritium-based auto-destruct device was used to destroy the Romulan scoutship Pi after it crash-landed on Galorndon Core in 2366. (TNG: "The Enemy")
The Dominion mined ultritium from a rocky gas giant moon in the Gamma Quadrant, before converting it into Internment Camp 371. (DS9: "In Purgatory's Shadow") The Jem'Hadar employed ultritium concussion shells to bombard a downed Jem'Hadar attack ship captured by Captain Benjamin Sisko in early 2373. However, the shells were merely an intimidation tactic and caused no harm to the ship. (DS9: "The Ship")
During the Dominion War, an undercover team led by Captain Sisko destroyed the Dominion's primary ketracel-white storage facility using enriched ultritium, concealed inside an empty canister of white. (DS9: "A Time to Stand")