I don't think they SAY what Tal was doing? o.o WoWpedia says 'other than siphoning power we don't know wtf he was doing'; I'm assuming he was there to help handle the sudden uprising of LIVING Nerubians going HELL NO, THOU REANIMATED JERKS, but there's not really anything to confirm or deny that.

Dangit Blizz.

Now I'm gonna have to make things up.

(And I keep getting these mental images of Lana "NO CHEAT CODES" and Arthas is like "L2P")