I like the idea of lost magic and am quite curious to see how the healing and blood magic could be utilized in a ttrpg context. If it were a party member, maybe it would attract too much attention to use, therefore the player never truly realizes the potential of their power because every time they try to use this lost magic it alerts the BBEG, either by magical means or word of mouth spreading rumors of a party of adventurers possessing lost magic.
I noticed a minor spelling mistake when talking about Mask;
Mast wears a featureless porcelain mask that completely...
It's funny. (to those reading that are not aware, we are cousins) I've been hearing about Mach'Lithe for over a decade, but never did really get my head around the severing of it from the rest of the world. I always thought of it as more of a Terry Goodkind ghost wall. Even though you described it as looking like an extremely powerful storm. I always figured that if you crossed the "wall" you'd make it to a normal continent that's just severed from the rest of the world.
I definitely have pre-planned consequences for using the lost magic if someone was able to find a way to do so! Some would be easier to avoid really being noticed with than other types. Some has limited knowledge still too of course - Demonology wasn't extinguished, just is now limited to some basic summoning and making deals with demons. Others like blood and healing are so well forgotten that both would draw a lot of attention to use.
And yeah, Mach'Lithe, depending on when a person crossed in, could be awesome and still high-magic... or it could be a horrific prison.
Part of the inspiration for the kingdom was actually from the old Darksword trilogy, though I made the setting smaller and a potential conflict much more of a problem.