This definitely does not seem to be Arthas/The Lich King speaking, notice how he says "For the Lich King's gaze is fixed upon you", last I checked that's not first person.

And didn't we have heralds come for previous events? C'thun didn't exactly walk up to Stormwind and say "hi".

"last I checked that's not first person."

Yeah, because crazy evil villains NEVER refer to themselves in the third person. He also clearly says MY minions, which is first person. So which is it?

No way to know anything until it happens. Anything else is pure speculation.

Yes, because it can't be Kel'Thuzad(or someone else really high up) calling some of his minions "MY minions", can it?

This could possibly be the voice of Ner'Zhul, and not Arthas.

Don't forget, as soon as Arthas put the Lich Kings armor on, they became one, more so I'm thinking the Lich King(Ner'Zhul) took over Arthas's body.

That's not really the way Ner'zhul sounds, and it's a bit odd that Arthas' voice isn't included with it. It's maybe a placeholder, but more likely, it's a different character.

Since the event is going to be a new Shadows of the Necropolis event (they already added the new quests for Silvermoon and the Exodar with slightly altered quest text), it's probably a character from Naxx or perhaps Darion Mograine. It sounds more like a lich, the way Kel'thuzad sounds, than the way the Lich King sounds.