Scrab

The Scrab looks like a huge crab with a 6 foot diameter body. The scrab is protected by a natural form of anti-magic shell. In order to affect the scrab with a spell, the caster must make a normal attack roll; on a hit, the scrab is affected by the spell (but still receives a normal saving throw if the spell allows one). If the attack roll fails, the spell is reflected back onto the caster.