I do believe Darth Nihilus used Force Drain on Darth Traya , being to be honest if you play through KotoR II it can be sensed if Kreia wanted at any point to hand out some WOOP*** she probably could .
I think Force Drain , kept her from counter attacking Scion as he was literally Man handling her .
To show my believe that Nihilus might not have any form of Force Sever is because at one point he is confronted by Scion who was in attempting to kill Nihilus . If Nihilus had Force Sever he wouldn't have just fried Scion , he would have likely full out killed him and shorten a otherwise very good story.
Side Note : 30+ Dollars later in Coins to Packs and no fricken mask of my Favorite Darklord Period . I feel I am being very much trolled by this game and very upset ..................
Tomorrow I will have spent 50dollars into coins , and if I do not get my Mask I will RAGGGGGGGE over this !
Actually, Nihilus did use Sever Force on Traya when he and Sion rebelled. Why he did not use it on Sion, I do not know. Perhaps whatever humanity still remained within him felt no need to do so. This could explain why he didn't outright kill Traya. Or perhaps he was using Sion to draw out the Jedi. I don't know why Nihilus would let Sion leave.
And you probably wont get the mask. Very, very, very rare piece.