Tau was the leader of a group of thieves whose ships stole USS Voyager's main computer processor, a holographic mobile emitter, and other items right through Voyager's shielding in 2374 using a stolen translocator device.
Tau sold his stolen items from a province he controlled on a Delta Quadrant planet. He dealt mainly in weapons, and if a buyer balked at his price, he would threaten to sell the weapons to the buyer's enemies. One of the items he stole from the Voyager was the Leonardo da Vinci hologram. Tau became Leonardo's "patron" and sold his inventions to the highest bidder. Captain Kathryn Janeway discovered his location, rescued Leonardo, and recovered Voyager's stolen property. (VOY: "Concerning Flight")