Celestial weapon bind

This deeply guarded ritual, known only to those of the most devout and virtuous hearts, allows the caster to bind the soul of a defeated fiend to the weapon that defeated it. if it was defeated by an unarmed attack the arm that killed it must be used, if a spell killed it the spell casters hand must be used. Both the celestial being and the caster must willingly agree to this dark pact. The spell cannot bind demon lords, arch devils, or demoniac gods. and depending on the celestial's and fiends power the spell will be more difficult to preform (the DM chooses what challenges you might face)

To successfully bind a fiend, the creature must have been defeated within the last 24 hours, as its soul is in the transitional phase between the mortal realm and the Fiendish Planes of Averkan.

Effect: The caster performs a one-minute ritual that corrupts the celestial-bound weapon with the essence of the defeated fiend. The celestial being bound to the weapon is consumed in the process, sacrificing itself to ensure the fiend's essence is sealed. The weapon becomes irredeemably corrupted. The bound fiend's soul, now trapped within the weapon, can corrupt the hearts of those who attempt to wield it.

Once the ritual is completed, the weapon is often hidden away by holy orders to prevent misuse, as its very presence can draw the attention of cults and malevolent beings. The spell's duration is effectively infinite if the binding is successful, and it is considered a permanent magical effect.
the spell can only be undone by wielding the weapon itself.
(a custom magical item should be made by the Dungeon Master if this spell is ever used)