Note: This spell first appeared in the Hellbound: The Blood War boxed set, and were designed for fiendish spellcasters. DMs must give specific permission for PCs to have access to these lower-planar dweomers.
In an attempt to develop a spell that'd be particularly effective against their enemies, the tanar'ri or baatezu came up with acidic blast. It hardly matters which side invented the spell - the magic was promptly stolen or copied by the other, and now each force has access to its power. Until recently, both races guarded the spell desperately from nonfiends, since they believed that it wouldn't do to give the celestials or mortals such a powerful weapon.
See, acid is fully effective against both baatezu and tanar'ri, and acidic blast conjures forth a bolt of powerful, caustic fluid that emerges from the caster's hand with great force. The acid shoots in a straight line for up to 100 yards, and causes 3d6 points of damage to the target (no saving throw allowed). The splash also inflicts Id6 points of damage to any sod within 5 feet of the target (a successful saving throw indicates no damage).
Unfortunately for the fiends, an intrepid planewalking mage stole a baatezu wizard's spellbook, fled to Sigil, and deciphered the spell before the fiend could reach her. The spell is currently circulating among Cager wizards, most of whom will be only too happy to share it with any who ask (for the right garnish, of course).