I thought it was Vecna who reordered the multiverse. (or possibly even just the side-effects of whatever he did to break the wards on Sigil and the Demiplane of Dread that reordered the multiverse)
And being able to strike down anyone who speaks their name or holds their symbol is a bog standard divine power. Hand of Death doesn't even have a prerequisite.
EDIT:
And what exactly happened with Aoskar? I always he assumed that he was struck down while trying to enter the city.