Munchkin: Imaginary Frenemies
You can't clear thousands of dungeons without making a few frenemies...so that's what we did! As long as your imaginary frenemies are safely stored inside their blister, they help you in combat — but if you release them to get at the cards inside, you can't use their rule anymore. Oh, temptation!
As long as the Munchkin Imaginary Frenemies package remains unopened, you may keep your imaginary frenemies in front of you as a +2 combat bonus. It cannot be lost to Curses, Bad Stuff, or theft, and stays with you even if you die.
At any time, you may open the package, releasing your frenemies, and draw as many Treasures as there are players in your game. Keep one of the Treasures for yourself and give each other player one each. Sign the two Door cards inside, and put them in your hand as well. When you play a card that you have signed, it has double effect. Save this card so you remember this rule in the future!