Magic Shield absorbs the first point of damage that would be otherwise dealt to an adventurer this round. That means:

• It applies to the first attack in the Monsters (or Ghosts) Phase this round. This attack deals one less damage. (An “Attack Everyone” attack will deal one less damage only to the adventurer in the front; the other adventurers get full damage.)
• If there was no attack, the spell reduces fatigue in the Conquering Step.
• If there was no attack and the player prevented Conquering this round, it can even reduce damage from the Poisoned Dart’s special effect at the end of round. (It might sound strange, but the spell finds a way, as it gets really bored when nothing happens.)
• Otherwise (if there was no damage dealt after the Fast Spells Step this round) the shield vanishes without effect.