Sadlygrove Tristepin will be able to keep the sword and all its powers. It will, however, be empty, as Rubilax has a body and a personality and can be apped by anyone. Fortunately, Tristepin can seek out one of the Burning Legion's weaker demons (as they are very plentiful) and imprison one of those himself, if you want to do the crazy ogre hulk thing and not have Warcraft paladin powers for him.

I mean, come on. Who doesn't want to fuse with a babe like this-- oh you probably mean this, I mean obviously that's probably the most fitting kind of demon replacement...