The card dropped by the shining fairy gives 100% poison immunity, when tested against the normal faries, it gives you both immunity from the debuff of getting poisoned, and the initial hit of the skill which deals poison damage. This works in the boss as well, the effect of Ephenia's 1.5k poison debuff doesn't get applied. But for some reason, the poison mist, which is supposed to be a poison damage attack, still hurt my character.
