settrus is not settra.
the tomb kings had their role in the old world as cradle for necromancy and as an esotic faction; settra being the quintessential expression of what should mean being a tomb king.
but they can't have the same role in aos; nagash already exists and is the dominant force of the underworlds, he wouldn't allow the tomb kings to exist as minor entities and them being a major entity would go against what's been established since the beginning; the ossiarchs already cover the aesthetic of the ancient-looking army while the rest of the death alliance has nothing to do with them.
tomb kings don't need to exist and the recent fanboyism around them is almost exclusively built as reaction to their misfortunes, like with bretonnia.
the best ways to fit tomb kings as a culture in aos? the messy ascension to godhood in the old world meant nagash wasn't able to wipe the previous underworlds completely clean, the tomb kings having the biggest culture revolving around the underworld and afterlife will mean that they would get to be the first important settlers and culture of shyish, influencing other civilisations and spreading their culture and knowledge, collaborating with their dead instead of being cursed into such a state, this time around; settra would have its own promised pocket underworld where his body is pure gold, because that's what his will for his afterlife was, and everything would be fine, for a time.
specifically until at some point in the age of myth, the great necromancer was freed by sigmar as a questionable mean for his great ends; nagash will crush the tomb kings, again, conquer shyish and eat the gods of the dead, again.
whatever pocket of resistance from the original underworlds and tomb kings could remain would flee and hide in the absolute deepest and less accessible levels of shyish, waiting for a chance to strike back, knowing nagash will have plenty of enemies and lands to conquer before being able to find them.