So here’s a setting concept:

Dungeons are a kind of Architectural magic that anyone can do (martial or spell caster). The gods did it yadda yadda. You have to design the dungeon yourself but any number of people can help you build it. Once built is has to be stocked with monsters, though some will naturally come to it.

The monsters are basically a power source, their day to days slowly charging the magic of the place and their spilled blood becomes the trigger. So once you start conquering a dungeon, you can’t back out lest all the stored magic dissipate.

Some Dungeon spells are extraordinarily potent, being the equivalent to Tomes for ability scores or granting a bonus feat. Some might might be a means to place a powerful enchantment on an item.

Like tomes, dungeons eventually recharge and can be conquered to receive the same benefit again.

To be such a dungeon there has to be at least 5 rooms connected by one or more paths. The simpler the structure, the weaker the boon.

Mages of this world are largely responsible for the particularly bad disposition of many monsters and certain races. Increasing their breeding rate to unsustainable levels, forcing them to raid or engage in cannibalism in order to supplement food supply has made them easily targeted villains. This makes it easier for “civilized” people to justify exterminating them for greater strength or intellect or a magic item.

Most Dungeons are known, most have an understood effect on completion and Adventurer’s have to compete for or buy the right to delve. The players have their own motivations for trying to conquer them, but toward level 10 they should be designing their own dungeon.

I realize in hindsight this is a riff on MAGI. Well it was good then; it’s good now.