I’d like to dive into the mechanics a little.
Assuming all PCs are generals and you are trying to use D&D 5e, they would obviously be a magic class. So their bloodborne spells would function as cantrips.
It makes sense that the rest of their spells would be available to them based on their factions. So if you chose to be a Songhai general, you would only have Songhai spells available to you. In most cases, the spell level could correlate to mana cost.
The only exceptions to the mana cost- spell level correlation would have to be the legendary spells. We would need to find some kind of limitation. Maybe we can say the PC must find certain components or only be able to cast it once per day or must use multiple spell slots.