Demon's Feather Pen

World Guide - Side II Book Entry
A quill pen bearing the name of the high ranking fiend that is the incarnation of corruption and contracts, the "Demon". It possesses a sinister appearance unbefitting of a quill pen and is clearly a tool of the mamono realm, constantly emitting a faint bit of mana, and if you recognize it and seize this pen, you will possess an item endowed with an unworldly "Devilish Charm".

As the name suggests, this pen, filled with the power of contracts possessed by Demons, is an extremely rare magical tool made only by the hands of "Arch Demons", a high ranking species with tremendous magical power rivaling that of the Mamono Lord's daughters, the Lilim. The arrangements written and exchanged with this pen become eternal contracts that can never be broken.

As far as I know, this pen isn't used very often, but in the current Lescatie, it seems the Demon's quill pen is mainly used when Lord Castor is performing his duties as a diplomat, together with Mamono Realm Ink in hand when writing documents for diplomacy, and is mainly used when concluding official documents and treaties that document agreements between nations. Most of the content exchanged is basically a peaceful treaty that benefits both nations, with many vowing eternal friendship with Lescatie.

The contracts signed by this quill pen will be managed by a large number of Demons, and those contracts will not be the type that is "unbreakable by magic", but rather the Demons, by their own hands, "will not let it be broken". For example, if you try to void an established contract, you will be given a warning by a Demon that appears out of nowhere, and should you still wish to break the contract, plot to outwit the Demons, maliciously tear the contract, or attempt to tamper with the document etc., then the contractor immediately becomes the property of a Demon, and it is said they are taken away to the "Underworld" where Demons lurk.

In fact, an anti-monster state that launched a surprise attack against a monster state it had signed a peace treaty with was immediately covered by a legion of Demons, and now it seems to have become a mamono realm ruled by Demons and Devils. In other words, a contract signed with the Demon's quill pen is one that is mediated by an agency managed and operated by Demons, and if the contract's contents are something the Demons aren't pleased with, it is likely the contract will be rejected in the first place.

This is a complete digression, but in marriages in the current Lescatie, in order to make an eternal contract of continuously loving their partner and exchanging pleasure every day, it seems this Demon's quill pen is used to engrave a pattern on documents swearing each other's love, and in some cases even on their partner's body, but since the promise between a monster couple is never broken in the first place, the Demons, despite being uninterested in this diplomacy, will sometimes bless them with a hint of jealousy.