Darth Nihilus is a member of the Sith Triumvirate in Star Wars: Knights of the Old Republic II: The Sith Lords (2004), alongside Kreia and Darth Sion. He consumes life through the Force on a planetary scale, and the game is explicit that there is no longer a person underneath: the mask and robes hold together an appetite.
Katarr
His defining act happens before the game starts. Nihilus arrives at the Miraluka world of Katarr during a gathering of Jedi and drains every living thing on it. The one survivor, Visas Marr, becomes his servant and later joins the player's party, which is how the story gets to look at him from the inside without ever making him articulate.
Deliberately unwritten
Nihilus has almost no dialogue, no backstory delivered on screen and no stated species or homeworld — this dossier leaves both fields blank rather than repeat a guess. That emptiness is the design. Where Kreia is an argument and Sion is a grievance, Nihilus is what the game says happens at the end of both: the dark side as a metabolic process that has outlived the person conducting it.
The design outlived the game
The white cracked mask is one of the most reproduced pieces of Star Wars visual design outside the films, which is a strange fate for a character who speaks perhaps a dozen lines in a language the game does not translate. He has become shorthand for a whole register of Sith — the silent, ruinous, faintly cosmic kind — that the films have never actually used.
The fight
He dies aboard the Ravager over Telos IV, and the encounter is written to undercut the myth: for all the scale of what he does to planets, in a room with people who can resist him he is a finite thing that can be finished. That deflation is the point the game is making about power.
Canon status
Star Wars Legends. The Triumvirate and the events of The Sith Lords have never been brought into current canon.
Reading note
Take him with Kreia and Sion or not at all. The three are one idea split into three bodies, and pulled out on his own Nihilus is a mask with a good silhouette.