Giant slugs live in dungeons and other underground complexes where they can avoid sunlight. Since these creatures are boneless they can squeeze through narrow openings and navigate around most obstructions. They use their sharp tongues to burrow through wood and hard ground. Giant slugs have a nasty bite, but their most effective weapon is their ability to spit acid up to 100-ft away. The first shot is almost always a miss (only 10% chance to hit), but this attack serves to approximate distance. After the first shot the chance hitting is base 100%, going down 10% for every 10-ft distance from giant slug; thus at 20-ft away the chance to hit is 80%, at 70-ft away 30%, etc. Because of their tough, flexible bodies, non-magical blunt weapons do no damage against giant slugs—only edged, piercing, or magical blunt weapons can harm them.

These creatures are usually a pale light grey with a white belly, but can be brown or black.