You could limit it to being usable once every other round. Being able to use it only on a single attack per round already limits it somewhat.
It's on the strong side, but then again a fancy shadow sword is the iconic power of the class, so it should be. I'm ok with it.As for the Umbral Power - sounds good. Is the progression alright though?
This ability is gained at level 12. These are just a few of its competitors. If anything, 2 Strength drain per attack is quite weak. I considered recommending 4.Wouldn't thr strength drain be a bit much? Full attacking with some extra attack stuff and extra actions could make that so horribly powerful.