Ghastly Charcoal Hexmallow

Marshmallow Magic This marshmallow has Has a hint of vanilla flavor but tastes mostly of burnt charcoal. After eating it, it turns your tongue black for a few minutes.
  You can use a bonus action to eat this marshmallow, when you do, your abilities, attacks and spells ignore any resistance to Necrotic damage a target may have until you finish a long rest. However, for the duration you also are vulnerable to Necrotic 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.