"We must follow the rules, and the rules state clearly that those people whose names come out of the Goblet of Fire are bound to compete in the tournament."
-- Barty Crouch Sr. (GF17)
This spell or spells makes a contract magically unbreakable.
References from the canon
- Placing a name in the Goblet of Fire constituted a binding magical contract. The people whose names were chosen were obliged to participate in the contest Even Dumbledore couldn't undo this magic, since Harry was forced to compete in the Tournament when his name came out of the Goblet (GF16).
- Snape swore an Unbreakable Vow to Narcissa Malfoy with Bellatrix Lestrange as Bonder (HBP2).
- The Weasley twins tried to get Ron to make one when he was about five, but Arthur caught them at it (HBP16).
Commentary: In a way, Petunia Dursley sealed a binding contract when she took baby Harry into her home after Lily died (PS1). She agreed to give him "house space" with her family until he came of age at 17 so he would be protected by the "bond of blood" they shared as family members. When Dudley was attacked by a Dementor and Vernon wanted to throw Harry out, Dumbledore sent a howler to Petunia that shouted, "'Remember my last, Petunia," reminding her of the original letter he left with Harry and her promise to help protect him (OP2). And she honored the agreement by telling Vernon that Harry had to stay with them.
Another type of "contract" that might be overlooked in the books is the agreement that Sirius Black made to be Harry's godfather. During the christening ceremony, a godparent stands up and agrees to look after a child in the absence of his parents.
If Sirius had married… Sirius was too busy being a big rebel to get married. When Harry was born, it was at the very height of Voldemort fever last time so his christening was a very hurried, quiet affair with just Sirius, just the best friend (EBF).
While Sirius was unable to keep his promise during his time at Azkaban, he tried to make up for those years when he escaped, donating his house on Grimmauld Place as the Headquarters of the Order of the Phoenix, and leaving both the house and the house-elf Kreacher to Harry when he died.
The Fidelius Charm works something like a contract between wizards with an agreement/promise to become someone's Secret Keeper, as Dumbledore was for Grimmauld Place until his death, and Peter Pettigrew was for the Potters.
Sirius told Peter he believed the situation should have worked like the Unbreakable Vow:
"THEN YOU SHOULD HAVE DIED!" roared Black. "DIED RATHER THAN BETRAY YOUR FRIENDS, AS WE WOULD HAVE DONE FOR YOU!" (PA19)
But there is no "death clause" in the magical contract for the Fidelius Charm, and the magical seal relies on the Secret Keeper being loyal, trustworthy and brave. Unfortunately, Peter Pettigrew was none of those things, and he sold out the location of Godric's Hollow to Voldemort. Lesson: if you want to protect your family, only trust your family. Arthur Weasley was the Secret Keeper for his own family at the Burrow, while his son Bill was the Secret Keeper for Shell Cottage, protecting his wife Fleur and everyone else who was staying there (DH24).