Raid boss summoned after a successful siege

Jakeman

lets say theres a ancient altar under a node and it needs souls of the fallen to summon an unspeakable horror. what if a unspeakable horror raid boss got summoned after a successful siege? your all cheering with your buddies that you won but then all of a sudden the skies darken and a ancient voice speaks out and now your faceing down a giant monster or do you guys think its a waste of dev time for something like that?

Ludullu

I feel like this system covers the content you're suggesting
https://ashesofcreation.wiki/Events