Darth Tenebrous is a Bith Sith Lord, master of Darth Plagueis, and the opening figure of James Luceno's Legends novel Darth Plagueis (Del Rey, 2012).
The Sith as a research programme
Luceno gives him a temperament nobody else in the lineage has: Tenebrous is a scientist, a starship designer by trade, and he approaches the Sith succession as an engineering question. The dark side is a system to be characterised. His apprentice is a component with a known failure mode.
The spare
Which is why he violates the spirit of his own order. Tenebrous secretly trains a second apprentice, Darth Venamis, also a Bith, as a hedge against Plagueis proving inadequate. It is a rational contingency and it is precisely the kind of hedging the Rule of Two exists to forbid, and Plagueis takes it as the insult it is.
The first three pages
The novel opens with his death — Plagueis kills him on Bal'demnic in the first scene — and then spends four hundred pages showing you what he built. Starting a book by killing the character who set everything in motion is a confident structural move and it works.
Canon status
The biography told here is Legends: the Bith species, the Venamis experiment and the death on Bal'demnic exist only in Stover's short story and Luceno's novel. The name, however, survives the 2014 reset. Current canon records Darth Tenebrous as the Sith Master of Darth Plagueis, and the Sith Eternal's 26th legion of Sith troopers is called the Tenebrous Legion in Star Wars: The Rise of Skywalker: The Visual Dictionary (2019). So Tenebrous is not a Legends-only character — he is a canon name with a Legends biography attached, which is a distinction worth keeping straight.
