[Black Leviathan]

Dungeon Fantasy is a weak platform to build a game that makes sense on. The setting has no organic basis to support the existence of any of the classes. The economics of that kind of world would be untenable. Villages or even towns would just collapse from mountains of coins washing over them from randomly timed dungeon crawls. I'd suggest you build something from Dungeon Fantasy instead.

If I was going to create a justification for underground complexes I'd start with the homes of underground dwellers overtaken by monsters or disease or other problems.

If was going to create a world with cyclic apocalypses I'd base in celestial alignments. That way you could have several years or even generations before the end of the world. The alignment might awaken a sleeping breed of monster that's particularly nasty or open a gate to another world with a rival expansive race like Orks.