_Listen carefully to each of the vision's voices, all except Sara's and Yogg'Saron's._ _It's mainly notable when you listen to Neltharion's voice, but if you listen REALLY closely, you can hear Yogg'Saron's voice behind them, that's pretty cool~_ _Further signs that these are illusions~_
Tennoshin Frieza if you followed closely back then yogg saron is foreshadowing events in the past, present and future. “His brood learns his lesson before too long.” He’s talking about wraithion not succumbing to yogg saron’s corruption which made deathwing and malygos both go mad. Arthas being defeated by you, the champion and Garona assasinating llane.
It somewhat spoiled a few events from ICC but we already knew about Deathwing and the creation of the Dragon Soul as that was already in the books. (War of the Ancients trilogy)
Someone already pointed this out in the comments, but, if you listen closely you can hear yogg sarons voice when neltharion speaks. Just further showing how nelth was being manipulated.
I dunno why they didn't stick to one voice actor per character through out the game. kelthuzad sounded different in vanilla and so did the lich king when they had that little meeting before u opened the door. also Lich king doesn't sound the same through out the WOTLK expansion. if you remember his appearance at the argent crusade you know what I'm talking about.
So kelthuzad's original voice actor (wc3 and vanilla wow) happened to be Lich King's voice actor. So you could see why they changed kelthuzads voice actor so he didn't sound the same as lich king.
Justin Gross, the voice actor for Arthas in W3, is still alive. They just made Kethuzad va, who is also still alive...also voice The Lich King in WoTLK. Justin kinda did off and on voice acting so death is not the cause
The Lich King in Vanilla was a meld of Arthas' voice and presumeably Ner'Zhul's through the helm but by the time of Wrath it was established they successfully fused or Arthas took over or whatever.