[Underground zone layer]

Cavern system.

NPC humanoids Dwarves (forts an mines), Goblins, Trolls, Knockers, Kobolds, Spiders, Ooozes an Slimes, Rock elementals/golems, etc.

1st. - Make the underground cavern zones have 1 or more surface access point to each above ground zone. At minimum 1 access point per top side area.

2nd- Make the Underground Cavern zones non-habitable for Castle building.

3rd - Give the Underground Zones their own lore as a sort of living entity. Which reshuffles it's layout every X amount of days, quarterly per year, or a specific moon cycle. V-Bloods lairs retain a static layout. Which can be shuffled. The Cavern zones themselves get a procedural layout shuffle. FOG OF WAR enforced every zone shuffle. This way going underground requires new exploration pathing.

4th - Players who are underground (either offline or online) at the time of the "reset" are returned to their coffins, the starting zone or any area that is PVP disabled.