Underhand Shield

Created by the mighty underworld magician Lorto, who deeply feared an attack by assassins, this spell gives you protection against attacks that you can’t see coming.  

Once cast, an invisible aura is formed around you. You get a bonus of + 5 to your AC against weapon and spell attacks that are rolled with advantage. Advantage that is granted by the factor that you can‘t see the attacker.  

Additionally, while having the protective aura, your enemies can’t score a critical hit upon you.    When you are being under attack, your aura flashes a blue light when being hit, thus being visible for a short moment.