What:



Stationary "guardian" enemies that can be found fairly deep (4+ levels?) and fairly far away from the crystal. Activating the guardian by having a hero in the same room periodically spawns waves of enemies (that behave just like regular waves). Killing the guardian unlocks an exit from the room that would otherwise be inaccessible, in rooms behind the door there's a fair chance of getting better loot than average. (Other option would be to just have it drop a chest when it dies). Leaving the room before the guardian is killed stops the waves spawning, but the guardian regenerates fairly quickly.



Why:



It would be something fun to find in the dungeon.



It would create some nice risk/reward considerations, like go for it right away, open some more doors before to get stronger/set up better defenses, or skip entirely.