An area with a wizard above a dungeon who sells keys. The keys are
of increasing value and quality (ex 10k:gold key) and upon purchase,
the player delves into a random gen'd single instance dungeon. It has 10
floors and each floor has chest or chests of appropriate value. However,
upon entering, the player starts suffering effects of poison or whatever
time limit coder decides to impose. The goal is to get to the level you've
paid for (gold chests on floor 1), find the treasure and make it back out
alive. 1 key is purchasable by boot or.. whatever cooldown the coder
decides.

In terms of explaining, i thought either the mage above the dungeon gives
the player a temporary water breathing spell and the player dives down
into a flooded dungeon and starts to drown ( replacing the poisoned
concept) or the wizard has found keys to a ruin and temporarily opens a
portal to the past where the ruin is.. still intact..tho time reasserts
itself after a while.