Acid is a somewhat rare non-physical damage type, appearing only in Sundered Lands and on Slag champions in Shattered Peaks. Because of its rarity, few champions have resistance or immunity to it.
Acid used to be even rarer before Draksar Morph and Myx Sandsnipe were released, only appearing on a few champions. However, since then, it has appeared on several champions; now there are enough to make an Amplify: Acid deck and boost acid damage using Myx Spitter, Skywing Acidcraw, and/or Dragon Skull.