Acid Sheath

You enclose yourself in a fluid sheath of acid that does not
harm you. You can breathe normally and cast spells while the
acid sheath is present. Any creature striking you deals normal
damage, but at the same time the attacker takes 3d8 points of
acid damage. Reach weapons do not endanger their users in
this way. The acid sheath does not protect you against other
attack forms, such as fire. If you are in a grapple, other
creatures in the grapple takes acid damage once per round at
the beginning of their turn. Any spell you cast that deals acid
damage while the sheath surrounds you deals an extra 1d8
acid damage.