Toggle for limiting the number of a specific mob.
Add a toggle to mobs to limit the number that can spawn in a room.
That in combination with a permanent flag for a creature could offer a few different choices when designing a room.
1. It could add a bit of flavor to a hunting location. Have a special named mob that has a real low spawn chance. When it does eventually spawn, the mob is perm and must be dealt with in order for a normal hunt to continue.
2. Could offer a bit of variation to slowing down hunting zones. For example: A good hunting spot for thieves/rangers. The special mob to spawn could end up being undead, for obvious counters.
Might be worth looking into mob health regen on permanent mobs. In this specific case, there could be an interesting take if the mob did not regen. Or if it was tied to the normal ticks characters get, that would at least let them continue to progress on a mob if they have to clear out. (this could already be a thing, if so ignore it)
Last point, it adds a bit of flare to an area, as a form of mini boss encounter. Better XP/drop chance when defeated.