The Tox Uthat was a quantum phase inhibitor, a device capable of halting all nuclear fusion within a star.
Invented by a scientist named Kal Dano in the 27th century, the Uthat was almost stolen by two Vorgon criminals.
In order to prevent the Uthat from falling into the wrong hands, the scientist traveled back in time to the 22nd century, hiding it on the planet Risa.
By the 24th century, the history of the Uthat had passed into local legend. A Federation archaeologist, Samuel Estragon took an interest in finding the Uthat.
After Dr. Estragon's death in the 2360s, his assistant carried on the search, with financial backing from Estragon's sometimes associate, a Ferengi with ill intentions.
However, the assistant betrayed the Ferengi, traveling to Risa without him.
In 2366, the Uthat was found buried in a cave on Risa, and the assistant was able to throw the Ferengi backer off her trail with the assistance of Captain Jean-Luc Picard.
In the end, Picard destroyed the device, in order to keep it out of the hands of others who showed up to claim it for their own ill-fated ideas.