The Sith engine is a game engine developed by LucasArts. The game Star Wars Jedi Knight: Dark Forces II is based on this engine, and its expansion, Star Wars Jedi Knight: Mysteries of the Sith, is based on a slightly updated version.
How could I forget this thing? Dark Forces 2 was an epic adventure that bringed us the story of one of the best jedi ever known: Kyle Katarn. In the 90's this game was so amazing... now is not that good of course, but I still love it.