Magic Sword of Covenant is one of the magic swords described in Monster Girl Encyclopedia World Guide - Side II.
Guide Book Entry
Among the swords used in the current Lescatie there are many that are called "magical" swords. Lescatie's magical swords were originally considered "holy swords," a symbol of the power of the religion of the Chief God and her power against monsters. However, the widespread mamono mana dissipated their magic and reversed their nature: now these "Magical Swords of Covenant" are a typical example of such "former holy swords". Among the monsters, the Demons are a symbol of the so-called "extremists" - a faction that seeks to turn the entire world into a mamono realm. They are well known for corrupting human men by entering into a "contract" with them, whereby men give themselves completely to these monsters in exchange for eternal pleasure with a special immoral flavor. In particular, the "Magical Swords of Covenant" are used by these very mamono. The holy swords, which were left behind during the Fall of Lescatie, have become monster weapons by contracting with powerful demons: they are now magical swords that contain their magical power. Originally, those who possessed holy swords could call upon the sacred power in the sword in exchange for an oath to dedicate themselves to the Chief God. In turn, these twisted swords, in exchange for a contract to dedicate their soul and body to the demon who originally wielded the blade, give those in whose hands they are held tremendous magical power, provided they fit certain physical parameters. These magic swords have a mysterious allure that attracts people, similar to that of the demons themselves. A man who has been enchanted by a magical sword and has coveted its great power will dedicate his life to a demon with a contract and accept the fate of a knight that will protect her.
Before the fall of Lescatie, these holy swords were a symbol of loyalty to the Church, given only to those knights who were recognized as among the strongest in the Order. It seems that their owners were told that if they would faithfully follow the teachings of the Chief God, abstain and strictly control their desires, they would further increase the power of these swords. But now that these swords have made a contract with demons, the reversed magical swords become stronger when their owner is colored by pleasure and debauchery. It is very ironic that these ex-sacred swords now play an important role in the expansion of the mamono realms, actively spreading depravity.[1]