Achilles Invulnerability

The spell can only be cast with the material components (a vial of the River Styx's water). After casting this spell, your body becomes coated in an impenetrable magical shield. Until the spell ends, you are immune to all physical attacks. Poison only affects you if you breathe it in or consume it. Poison cannot be absorbed through the skin while the spell is active. A small spot on the surface of your body (a circle with diameter 3 cm) is the only weak point, where the shield has no effect. You can choose where the weak point is located on your body. If the weak point is hit, you take double damage on that attack, and the spell ends. For example if an arrow is aimed successfully at the tiny weak spot and hits, you take double the final damage total, and the spell ends. This means that if you go into a pool of acid or lava, the weak point must remain above the liquid.