Alright, so how would you get it? Would it be a fourth subclass? Or Would it delete an existing subclass in place of a Darker subclass? Or would all your subclasses change into darker variations? If it is just a dark fourth subclass, what about those who don't choose the dark? What do they get? Or are they forced into the dark subclass?

I don't think a sub class should be removed nor changed but rather add one to each class. In the grimoire it is said that there has to be a king, we killed the current one oryx so I think there should be a mission where you find the worms and maybe make a bargain, have a choice to kill them in expense of not getting this sub class or bargain with them and get this sub class, just an idea but I think I could come up with something better then joining the worms, or maybe a mission where you venture into the taken realm and the only way to get out is to get this sub class and using it to kill the worms, i'm mostly spitballing ideas here xD

Perfect answer.