Explained in an extremely long and boring theatrical performance the top members of the family had to sit through, the celebration was instated by the Devils when the foundations of Hell administration were made in ancient times. Though only a few short scenes of the play are shown, the festival appears to be similar to marriage and is meant to give power to the magic users in reverence of the Devils.
When the time for the festival approaches, blue beetles begin appearing and flying around the world, heralding Blue Night (presumably named after the beetles). The festival es held in the En Manor, and one must qualify to enter, as well as to send application forms to the members of the En family or other magic users. The festival itself is very similar to a carnival or circus, with game tents, shows and food stands being built on the estate. The main event of the Blue Night is the Contract exchange to become partners with another magic user.
The event consists of two Magic Users that are to become partners heading into the Black House. Once there, a Devil (shown to be Chidaruma) will use a door handle-like item to open their chests and pull out their Contracts, special organs with the appearance of actual papers made of flesh. Then, the participants exchange and sign their contracts, whereupon they place it inside themselves. Once completed, the magic users become Partners. When done willingly, Partnership doesn't have a very tangible effect but appears to tighten the bond between the participants, which is why Partners are usually friends.
One can manipulate or even force another magic user to sign the contract. If done this way, the former partner will have a limited form of mind-control over the latter, where their personality is altered and they become subservient to their partner. This prompts many shady magic users to kidnap and sell powerful or rare sorcerers for this event.
The partnership will last until the next Blue Night (when the contract becomes blank again), if Chidaruma nullifies it or if one of the partners dies. If the contract is removed from the body of either partner, the partnership will be interrupted and the original contract's owner will feel the pain of the removal, however if the partners ever meet face-to-face again, the contract will materialize inside them instantly.
It is shown that even forced partnership can be "resisted" through immense force of will (as shown when Nikaido rejected En's contract), but this is incredibly draining and might kill them from the strain.
- The ritual is very similar to marriage, as the participants exchange vows (contracts) in a church-like building (the Black House) under a divine figure (Chidaruma and the Devils). However, romantic attraction is never brought up and the partners might simply be good friends (if not coerced).
- Plays into the trope of devil contracts
- Blue Night and the blue beetles feature prominently in the fourth ED of the anime, "Strange Meat Pie".
- The door handle appears to not be necessary for the ritual, but it makes it quick and painless compared to the forceful extraction of the contract.
- It is explained by Kawajiri that sometimes Devil Exams intentionally line up with Blue Night so that the trainee is targeted for kidnapping and they have to fend for themselves.