The Living Dungeon

Based off a DnD 5e homebrew monster I found and love to use.

An adventurer, or adventuring group enters a dungeon in hopes of finding treasure and fame. What they find is something completely different, and unexpected. The worlds largest Mimic. The dungeon itself is the Mimic. Starting out as a simple chest, as it killed and devoured more and more adventurers, it slowly evolved, becoming the dungeon it originally called home. 

It can create monsters, alter the layout of rooms to confuse and separate adventuring parties, turn entire rooms into instant death traps, and so much more. In this scenario, the Mimic would've recently eaten an entire army, that had been on a march and training exercise, so it decides to leave the adventurer/adventurers alone and let them explore. The monsters that decided to make the Mimic their home, as well as the simulated monsters it created have another idea, instead of killing them, taking them captive as pets and using them as property to trade with the other monsters. 

I'll add more as time goes on, adding more to the idea if I come up with more.