As a reaction, which you use when you are targeted by a spell, you gain advantage on the saving throw against the spell. If the spell only targets you and you roll an 18, 19, or 20 for the save and the spell costs 3 or less mana, the spell has no effect on you and instead targets the caster, using the mana, spell save Difficulty, attack bonus, and spellcasting ability of the caster.