The Tox Uthat was a device that could stop all nuclear fission within a star.

It was invented by Kal Dano in the year 2658, but was sent back in time. Stories said that Jean-Luc Picard had discovered it on Risa so two Vorgons traveled to the 24th century during a visit by Picard to Risa. Picard and Vash found the device which the Starfleet captain subsequently destroyed. (TNG episode: "Captain's Holiday", Last Unicorn RPG module: All Our Yesterdays: The Time Travel Sourcebook)