This one would come under the category of Specials:
Quote:
Originally Posted by Rules
Other Treasure cards (like Go Up a Level cards) are “specials.” You may play these at any time, unless the card itself says otherwise. Follow the card’s instructions, then discard it, unless it has a persistent bonus like an Item.
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Now, as the bonus is technically not persistent (you only get it once you use it, so it is not giving you a bonus at other times), you should discard it after the combat in which it is used.