Oozes

The corridor was clean. Too clean. That was the first warning.

Oozes are mindless predatory organisms that dissolve organic and sometimes inorganic matter for sustenance. They are a ubiquitous hazard in dungeons, sewers, and ruins, where they serve as the ecosystem's ultimate decomposers. Their amorphous bodies allow them to squeeze through gaps that would stop any other creature of their volume.

Gelatinous cubes are perhaps the most infamous dungeon hazard in the known world. Nearly transparent, they fill corridors perfectly and advance slowly, dissolving everything in their path. The skeletal remains and undigested equipment floating within a cube are often the only warning of its presence. Imperial-era dungeon architects reportedly bred them as self-maintaining cleaning systems for underground complexes.