Darth Nihilus is a Dark Lord of the Sith who appears in The Jedi Masters mod.
In the 1.0 version, Nihilus returns as one of the main antagonists of the mod who's mask is captured by the True Sith and he is ressurected and so he allies himself with the True Sith and you later get to fight him during the Battle of Rhen Var. In the 2.0 version, his mask is captured by Freedon Nadd and he can ressurect Nihilus to replace the Exile in your crew.