Sylvanas will win. Why?
If I was flippant I'd say she has something the Deathlord doesn't, with or without Apocalypse, and that's plot armor.
Or I could say that if Sylvanas ever WAS destined to be defeated by the Deathlord, the Deathlord would need at least nine of their friends to accomplish it. Which means Sylvanas would beat them one on one.
But the honest truth is that Sylvanas is extremely skilled, clever and manipulative. She would win because no matter how good the Death Knight thinks they are, Sylvanas would be better.
There is no such thing as celtic inspired warcraft 2 high elves. You made that up.
High Elves share the exact same culture as Blood Elves and after they betrayed their people the few High Elves aligned with the Alliance didn't sit around a campfire going 'now that we are with the Alliance, we need to completely invent a brand new culture for ourselves for the sake of pretending to be different'. That's not how culture works.
If ANY group is 'celtic' inspired...and I say this as an actual Celt by the way, one hundred percent Irish and actually living in Ireland...it's the Dwarves.
What you mean by 'celtic' is to turn them into wood elves. Sorry, Night Elves already have that covered.
If High Elves are adopting any culture, it's Humans.
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Ha, I think this is probably the truth.