Gelatinous Cube
Creature
The Gelatinous Cube is the most insidious of the oozes and a thing of pure dungeon-horror: a vast, near-transparent cube of living acidic jelly, sized precisely to fill a corridor wall to wall, that slides silently along sweeping the passage clean of everything in it. It is so clear that an inattentive party may not see it until they have walked into it and begun, slowly, to dissolve; suspended within the larger old ones can be seen the half-digested bones and coins and blades of everything the corridor has fed it. It is mindless and slow, and lethal precisely because it is so easy to miss.