Just throwing out what might be a small, interesting difficulty tweak to spice up underground play. The dungeon frenzy would either be a delayed-onset event that triggers when the player stays underground for too long, or something that can be triggered via a trap, hazard, or item. When it activates, the dungeon's local mobs begin spawning faster, stronger and in greater numbers, with the frenzy slowly (or quickly) incrementing over time until it becomes impossible to survive. To incentivise engagement, DF also adds an incrementally increasing quantity of gold dropped per mob killed while it's active; however, if the player dies, all gold spawned by the event is destroyed when the map is closed (all players in the space are gone).