The Tox Uthat is a quantum phase inhibitor from the 27th century that can halt nuclear fusion inside a star, found by Picard and Vash on Risa in 2366.
The device was created by a scientist from the 27th century, who brought it back in time and hid it on Risa to keep it out of the hands of criminals. Two Vorgons from the future came looking for it, as did the archaeologist Vash.
Vash eventually recovered the Uthat, and after a complicated chase Picard took possession of it. To prevent it from being used, he destroyed it with a phaser, and the Vorgons departed with nothing.