"A very dangerous piece."
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 some-time associate, a Ferengi named Sovak. However, Vash betrayed Sovak and traveled 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 it using Transporter Code 14, to prevent it from falling into the wrong hands, thus fulfilling his role in history. (TNG: "Captain's Holiday")
In Q's Guide to the Continuum, Q says that the Vorgons would not have been a threat even if they had gotten their hands on the device; that in almost every possible reality where they found it, they ended up blowing themselves up with it. But then, Q does have a very strange sense of humor.
The Tox Uthat returns in the Star Trek Online episode "Sunrise". Players also meet its creator, Kal Dano, who came from the 31st Century to save his ancestors from their star dying and wiping out their race. However, it was a plot by the Tholians to obtain the device, who steal it when his ship is overwhelmed. The following episode "Stormfront" has the player character and Kal Dano attempt to retrieve the Tox Uthat, only to have it used to kill a star and threaten the homeworld of the Na'kuhl. The two time travel to the 22nd century and bury it on Risa for Picard to fulfill his role in history.
In the Agents of Yesterday story line, Agent Daniels reveals that Picard had in fact faked the Tox Uthat's destruction by using a transporter, and joins the player in taking the device from its secure vault beneath Starfleet Headquarters during the Breen attack on Earth in 2375. The Tox Uthat is then brought to the 26th century, where it is installed aboard the USS Enterprise-J to defeat the Sphere-Builders at the Battle of Procyon V.