The Tox Uthat (2366)
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, Ajur and Boratus. In order to prevent the Uthat from falling into the wrong hands, Dano traveled back in time to the 22nd century and hid it on Risa.
By the 24th century, the history of the Uthat had passed into local legend. A Federation archaeologist, Samuel Estragon, and his assistant Vash took an interest in finding the Uthat. After Dr. Estragon's death in the 2360s, Vash carried on the search, with financial backing from Estragon's sometimes associate, a Ferengi named Sovak. However, Vash betrayed Sovak and came to Risa without him.
In 2366, Vash found the Uthat buried in a cave on Risa, and subsequently threw Sovak off her trail with the assistance of Captain Jean-Luc Picard. Ajur and Boratus traveled back to this point in time, having discovered from historical records that Picard would locate and destroy the Uthat. Posing as security officers, they attempted to convince Picard that they were interested in securing the Uthat for safekeeping. However, when Vash informed him that it was two Vorgons who attempted to steal the Uthat in the first place, Picard destroyed the it using Transporter Code 14, to prevent it from falling into the wrong hands, thus fulfilling his role in history. (TNG: "Captain's Holiday")