Brilliant Grape Hexmallow

Marshmallow Magic This marshmallow has a tart white grape flavor. It glows softly after it has been exposed to sunlight or moonlight.
  You can use a bonus action to eat this marshmallow, when you do, your abilities, attacks and spells ignore any resistance to Radiant damage a target may have until you finish a long rest. However, for the duration you also are vulnerable to Radiant damage. If you are resistant, you lose the resistance. If you are immune, you become resistant. For the duration, you cannot gain the benefits of any other hexmallow.