Mistress of Souls is a title given to certain Myrkul-aligned spellcasters within the Cult of the Absolute. Mechanically, they are identical characters, bearing the same statistics and equipment, differing in personality, aesthetic, and function within the story.
When you are about to be hit by an enemy, use your Reaction to increase your Armour Class by 5. You also take no damage from Magic Missile. These effects last until the start of your next turn.