acid polymorph transmutation 

Traditions arcane, primal

Cast [>>] somatic, verbal

Duration 1 minute

You exhale corrosive gas, and acidic secretions coat your skin as you transform into a being of living acid. You gain acid immunity. Any creature that touches you or damages you with an unarmed melee attack or non-reach melee weapon takes 3d6 acid damage, and your unarmed attacks deal 1d4 additional acid damage. The first time each round that you deal acid damage to a creature in this way, you gain 3d6 temporary HP as your body digests the eroded material and transforms it into a protective acid. When the spell ends, any remaining temporary HP expires as well. Your acid.

Secrets of Magic, page 96.