Dungeon core.

You were reborn on a magic world as a dungeon core, given the ability to, with time, build out an expansive ancient dungeon filled with traps, monsters, gold and jewels with the ability to bend magic to your command within your territory. At least that's what the goddess told you before dumping you in a cave with nothing more than your dungeon core shard and a hand full of mana to start working with. (To start off with you would have an avatar, one suitably small and weak to go with your dungeon core, and a hand full of creatures that you can spawn in once a week. To expedite your dungeons growth you could choose to use your avatar  (Or other adventurers later on) as a breeder, pumping out more monsters than you could spawn and gaining the ability to edit each new generation while it grows within you.)