T'Sil was a female Vulcan in the 23rd century. She hailed from the Last-of-all-Cities on Darien 224, a lost colony stranded from the rest of the Vulcan people before the Logic-reformation, but lived as an outlaw, opposed to the colony under the leadership of Matriarch T'Kell instead allying herself with Tagok.
In 2254 Tagok captured the Starfleet starship USS Cortez which was surveying the planet. Six weeks later the USS Enterprise arrived in the system search for the Cortez and Tagok promptly attacked. He had T'Sil lead boarding parties in an attempt to capture the Enterprise but they were repelled and T'Sil was forced to order a retreat back to the Cortez. Not long after the Cortez with T'Sil still aboard was destroyed by the Vorl-tak a weapon fired by T'Kell and her followers from the planet below. (EV comic: "Cloak and Dagger")