Tarnis was a human male Sith Lord

Sith Lord Miscellaneous Ranks

of the reconstituted Sith Empire who lived during the Great Galactic War

Great Galactic War Event Great Galactic War (3681BBY - 3653BBY)

and the subsequent Cold War

The Cold War Event Cold War (3653BBY - 3642BBY)

with the Republic

Galactic Republic Organization Galaxy-Wide Organizations

.
The son of the legendary Darth Angral

Darth Angral (Human) Character Supporting Characters

, Tarnis was trained by his father's apprentice Lord Praven to the rank of Sith Lord before embarking on an undercover assignment not long after the Treaty of Coruscant ended the Great Galactic War. Posing as the Republic scientist Eli Tarnis, he infiltrated the Republic's superweapon research division

Superweapon Research Division Organization Galactic Republic (GGW, CW and GW)

and quickly became the head of the design team for the Planet Prison weapon on the Republic's capital Coruscant

Coruscant Planet

. While his initial purpose was to gather information from inside the Republic, Tarnis believed his cover was in danger in 3643 BBY when the Jedi Master

Jedi Master Miscellaneous Ranks

Bela Kiwiiks

Bela Kiwiiks (Togruta) Character Minor Characters (TOR)

arrived on Coruscant to investigate

Investigation on Coruscant Event Cold War (3653BBY - 3642BBY)

a "dark presence" the Jedi Council

Jedi Council Organization Jedi Order

had sensed on the capital. He made his move and attempted to steal the Planet Prison with the help of the criminal organizations known as the Migrant Merchants' Guild

Migrant Merchants' Guild Organization Planetary and Sector

and Black Sun

Black Sun Organization Galaxy-Wide Organizations

.
Staging his own kidnapping

Kidnapping of Doctor Tarnis Event Cold War (3653BBY - 3642BBY)

from the Senate Building after hiring a thief to steal

Raid on Military Storehouse Event Cold War (3653BBY - 3642BBY)

the design files for the Planet Prison, Tarnis fled to the territory of the paramilitary Justicars' Brigade

Justicars' Brigade Organization Planetary and Sector

, but was pursued by a young Jedi Knight

Hero of Tython Character Major Characters

. Tarnis' true identity was uncovered

The Rescue Event Cold War (3653BBY - 3642BBY)

by the Jedi Knight, working with the Republic Strategic Information Service

Strategic Information Service (SIS) Organization Galactic Republic (GGW, CW and GW)

, the Coruscant Security Force

Coruscant Security Force Organization Planetary and Sector

, and the Padawan

Jedi Padawan Miscellaneous Ranks

Kira Carsen

Kira Carsen (Human) Character Supporting Characters

, and the Sith was followed to the ruins of the Jedi Temple, where the Knight and Carsen faced the Sith Lord in a duel

Race to the Ruins Event Cold War (3653BBY - 3642BBY)

in order to stop him from using the Planet Prison against Coruscant itself.
Tarnis' death was witnessed via hologram by his father, who declared everlasting vengeance upon his son's killer and embarked on a crusade to gain revenge upon the Jedi.