Will-o-the-wisps: gray or possibly bright blue y-class monsters that spawn individually, leave no corpse, and have no attacks except a passive blindness attack. They emit light radius 0 so you can see them, but no surrounding squares, across a dark area. Not infravisible, and could have teleportitis to replicate their folkloric effect of suddenly vanishing. Spawn rarely in swamp rooms and may not spawn at all in a lit area. Most importantly, they have a special AI that causes them to avoid the player and orbit around treasure (occasionally) or around hazards like monsters or traps (commonly).

Use a miniature cellular automaton to determine which spaces in a swamp room will become water, rather than using a fixed checkerboard. This may not work well for tiny rooms, so enforce a minimum size constraint on what can become a swamp room if necessary.