Fluff and Puffs

Fluff is a recently-discovered threat in the Beneath. It is a gas that seems to fluctuate in density, hovering just above floor level in dungeons and caves, slowly migrating as if blown by an unseen wind. Fluff itself is reasonably stable, if just as insidiously in-the-way as tarstuff. It may block passages or monster movement, but it is inert and harmless by itself. It can even be pushed around or swept into a corner with a suitable implement.

The real problem is that fluff remains very cold and very poisonous. When a dense pocket of fluff forms in a single tile, either from being pushed or growing into a narrow space, the resulting Puff becomes much less inert. Owing to some thermodynamic instinct, the tiny gas cloud slowly seeks out the nearest heat-emanating creature, moving one tile every five turns, and tries to collide with the creature, smothering it in freezing gas (and incidentally freezing any nearby water in the process). Puffs ignore "cold" monsters like rock golems and seep, only seeking out creatures with a significant body temperature. However, they can collide with any creature by accident.

Fluff and Puffs emerge slowly from particular sources or vents, showing an uncanny synchronicity with the spawn cycle. Puffs can also converge together with other Puffs or existing Fluff to combine into more Fluff, which might block escape. Unfortunately, much like briar, there are only indirect defenses against it - explosives, hot tiles, the venerable yellow door, and a good pair of running shoes.