A huge crab with a 6-foot diameter body protected by natural anti-magic

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. Scrabs carry their eyes on armored stalks, which means that no bonus is awarded for attacking them from behind.