Goblet of Fire

From Hpwiki

During the Triwizard Tournament, three champions - one from each participating school (in this case Hogwarts, Durmstrang and Beauxbatons) - must be picked.

To make this process of selection completely fair and magically binding, all aspiring champions have 24 hours in which to submit their name into the Goblet of Fire - a large, wooden goblet, encompassing many dancing blue flames. For the tournament held at Hogwarts in Harry Potter and the Goblet of Fire, Professor Dumbledore draws an age line around the goblet, so that no pupil under the age of 17 (the age that young wizards and witches become "of age") may enter the competition.

Placing your name into the Goblet of Fire constitutes a magical and binding contract that the wizard or witch in question may not even break if they did not submit their name themselves, or even know about it.