Long story short: I got a little suspicious of Nihilus' babbling so I took the sound files and played them backwards - THEY MAKE SENSE!!!
Okay, here's what he says:
During the cutscene where we first meet him and Visas:
Nihilus: You have sensed him? The one who stole my essense? Find him and bring him to me. If you lie... remember - your life is mine.
On your encounter with him on the ravager:
Nihilus: You! Thief! At last I will be whole again... the essense... MINE!
When you tell him there are no jedi on Telos and this is all a trick by Kreia:
Nihilus: I did not come looking for Jedi... I came looking for you... and I have found you...
Nihilus: At last I will be whole... what you took from me at Malachor Five I will take back...
Nihilus: Stand with him, Visas, and you will fall...
Near his death:
Nihilus: No! Master... Traya... (or it could be Betrayal, I couldn't tell. The sound was fuzzy)
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I don't know why the devs didn't let us figure this out with a "Translator", but there was suppose to be an encounter with Nihilus on Peragus. The block, however, was commented out in the scripts.
This should shed some new light...
My take on it: At Malachor V, when you lost the force, Kreia said "Many more were born in that dark crucible", maybe Nihilus was the result of the massacre, a force spectre like that person who made the theory thread in storyline discussion termed it. Kreia led him to believe you were the one who took his "essense" when in fact it's the other way around. So this dude's been eating other people's force to sustain himself since he "lost" his essense. But when he tried to suck you, haha, joke's on him.