But why would Blizzard use Arthas' father as the main speaker of the ghostly host, it seems more than a coincidence, I highly doubt there is no consience or fear left in the LK if Blizzard use thoughts of the past as if the LK were plagued.
Well Diablo takes the form of fears, thats what he is. If his opponent is fearless then Diablo would technically would not be able to be percieved by the LK.
Also note the power levels, where Azeroth, the scourge and all those who have fought have only been fighting for what? hundreds if not thousands of years....the burning hells and the high heavens fight across "both time and space, infringing upon the very fabric of reality itself" "The great conflict burned hotter and longer than any of the stars in the sky" , taking this into account Diablo and his brothers are ancient beyond the Lich King, perhaps even more so than those that created him (Eredar).
Hells armies that foughtacross time and space as stated and who have fought for countless eons beyond most Azerothians and perhaps even the LK could imagine. The three brothers, Diablo, Mephisto and Baal slaughtered an entire third of hells armies when they were supposedly going to be exiled. This information is from the Diablo manuel, Diablo being the strongest of the three lords of hell will certainly be no push over for the LK.