I don’t know the ESO story, but it’s certianly possible that the Devs took theor inspiration from there. Most of WoW is in some way derivative of other worlds and stories, with Warhammer being the most obvious one, so there is no reason to rule ESO out.
That said… neither powerful villains that want to dominate, nor other planes of existence, nor hellscapes, nor villains being associated with instruments of torture, nor necromancy, nor royalty, nor turning henchmen nor the henchmen opening the way for their masters are in any way uncommon. Indeed, I would call all of these common tropes. That doesn’t mean that the combination here is common in itself… but let’s just say randomly combining fantasy elements you wouldn’t have to wait that long to get that story structure, and it could well happen twice independently.
I’ve played ESO quite a bit (over 300 hours for sure) and I must say I never thought of this (still haven’t gone onto SL), but yeah you’ve got a point, it is actually VERY similar put this way.