Sith undead were long-dead Sith warriors that lay in the catacombs of the Dark Force Temple on Dromund Kaas. Ghastly in appearance, their weapon of choice was Force lightning.
On alien presence, their rotten bodies were reanimated by the dark side aura of the Temple to protect it. Mara Jade faced these warriors during her quest for Kyle Katarn on Dromund Kaas.
Sith undead are not mentioned in the game's manual. This is most likely to make their appearance a surprise.