Ii'll preface this by not knowing that much about the lore of WoW but i'd break it down to A) Comparing general resistance of population of azeroth to tamriel B) Organized forces response C) Highly powerful individuals

A) Tamriel is the size of half of Australia apparently, at best. And has a population of maybe a couple of million(daggerfall is like 100 000 and it's one of the biggest cities). On the WoW size, stormwind is 200 000 and probably the biggest city, so it's somewhat comparable population wise i'd guess.

https://www.reddit.com/r/teslore/comments/19y118/the_lorecorrect_size_of_tamriel_20_pictures_inside/

B) Well, if it's Skyrim the game timeline. Then neither the Stormcloaks nor the Legion can really muster up an army to fight him as they're stalemated. Given how much less powerful necromancers(Potema Septim) nearly brought Tamriel to it's knees. And given how there's plenty of undead and necromancer for the lich king to enlist, i don't see a problem in him rallying an army.

C) This then comes down to the power and willingess to intervene from the divines side of things. The biggest feat of power that comes to my mind is the ministry of justice incident where a daedric prince sent a meteor at a city and Vivec stopped it mid air with the power of his mind. Not really sure how the lich king matches up to that, but yeah, that's about the strongest feat i can recall from TES.

D) Not really sure about this whole champion and merge with him business, as i said, WoW lore isn't really on point, just felt like trying my hand at this i guess >_>