Queen Led would make a fitting deity for the dead. Making a civ out of the overseers from the first three forts that attacks the next fort? Awesome.
Perhaps we could say that, in order to make ourselves perfectly good, we completely banished the evil from ourselves. That evil became our shadow selves, formed a civ, and started wreaking havoc on life, the universe, and everything. Unfortunately, that would require us to actually plan a fort's design so that it looks like a citadel of righteousness instead of a massive, chaotic clusterfuck.
Dwarven righteousness is a massive, chaotic clusterfuck. Also, I think it's unfitting(in this fortress saga at least) to limit the players to a certain method of gameplay and fort design just for the sake of plot. These things are better when they evolve on their own and through the individual approach of each overseer.
As a possible suggestion to the next fort and potential new foes, why not have the enemy compose of all the species who have had members killed in the fort, through accident or hostile intention? Dwarves, human, elves and goblins. I think kobolds are extinct though. Can't remember. Rare caste members which are just resurrected megabeasts and/or titans killed in previous forts as well perhaps?
Then again, such a feature would just be nitpicking so I'm not going to lie and say it's something to add because it'd be a great idea. Since I'm a perfectionist at random intervals though, if it was up to me, I'd make a hostile civ that composed of all previously mentioned species just because it fit some reason along the lines of "Finally unleashed, Queen Led's undead horde marches forth" or something ridiculous like that. It might be unneeded complexity though.