And wat do you mean the Soul reaver is an uber haxxed weapon?
It steals souls, probably with as little as a scratch and can cut through boulders, that seems like a pretty powerful weapon to me. Plus I've seen you wax lyrically about how good it is. The only thing I don't know is whether it would be able to kill kill Kain (does he even have a soul?).
And Dar'Khan's tk just seems more powerful then Kains. He's shown throwing 3-4 elves around with visible ease (the weight of 3 people > an armoured knight), without even looking at them or slowing down. Kain always gestures to use tk, whereas his opponent doesn't even have to look at his target, so Dar'Khan will be able to use it faster and with more powerful effects. Add to that his shieldspell which is powerful enough not just to block a knife, but to liquidate it, and his exceedingly fast reflexes and I can see him taking whatever Kain throws at him, physical-wise.