As others have already pointed out, his voice does indeed sound pretty dead. However, I feel it kinda suits the character.
But I think it can still use a bit of improvement. Of course all lines have been recorded already, so there's no changing that; but there's no need for that either. His voice is okay, there's just something missing, something out-of-this-worldly. He's a creature from another plane after all...
Larian, can't you add some kind of echo or other effects to the recording so that the DK sounds a bit more Darth Vader-like? I think that would really help a lot!
Just my two cents.
