Former long-time LucasArts' employee Ryan Kaufman (now at Planet Moon) writes up how the Jedi Knight expansion pack Mysteries of the Sith came into being in this two part entry on his starwars.com blog; Part 1 | Part 2
Mara finds and enters the ancient Sith temple and discovers that Kyle has gone a little bonkers with dark side power. She has to chase him to the center, but first--- ZOMBIES!
That's right. I don't know how we got this one past Licensing, but there is an area of the temple called The Catacombs, where the undead Sith will rise and fight you. Yep, Sith Zombies, people. It was great. They shot lightning from their hands and were generally pretty pesty. And it's canon. Zombies are canon...
We remembered how both Obi-Wan and Luke had dropped their defenses and sacrificed themselves in their conflict with Vader (who had been corrupted by the dark side, as Kyle had.) We realized that we had a great hook for our final battle-players would have to surrender to the possibility of utter defeat, power down their lightsabers, and draw on the goodness that was still within Kyle.
Nice to hear a bit more about that oft-overlooked Jedi Knight expansion pack.
Source: StarWars.com