Immunity to Acid

A creature with acid immunity never takes acid damage. Even when subject to spells or abilities that reduce a monster's acid resistance, they will continue to remain immune to acid damage.

List of monster types that are immune to acid damage

 * Will-o'-Wisp

PnP Comparison

 * In DDO, creatures with Acid Immunity don't necessarily have vulnerability (+50%) to ?. Some do, but many such as ?s do not.