Sweet Rose Hexmallow

Marshmallow Magic This marshmallow has a sweet, floral rose flavor with a delicate sugary finish. It smells like a lovely bouquet of roses.
  You can use a bonus action to eat this marshmallow, when you do, your abilities, attacks and spells ignore any resistance to Psychic damage a target may have until you finish a long rest. However, for the duration you also are vulnerable to Psychic 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.