This card is a Door with the fantasy Munchkin theme. It's a standard +10 monster enhancer.
Unlike most other promo cards, this one is not given out freely. To get a copy, you have to either (1) purchase five Munchkin booster packs from the publisher's online store, or (2) mail in the UPC codes from 5 boosters. The offer is good through December 31st, 2014 or until they run out of cards, whichever comes first. There is no other way to obtain this card, and any remaining cards will be destroyed after the deadline. (Note that the publisher printed 10,000 copies.) More information about this offer can be found here.