Echoing Elderberry Hexmallow

Marshmallow Magic This marshmallow has a rich, elderberry flavor with a hint of lavender. It vibrates in a soft pulse when you touch it.
  You can use a bonus action to eat this marshmallow, when you do, your abilities, attacks and spells ignore any resistance to Thunder damage a target may have until you finish a long rest. However, for the duration you also are vulnerable to Thunder 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.