The Purifier is a sentient, mildly telepathic weapon oil. It comes in a small, flawless crystal bottle with a flawless crystal stopper which refuses to open for anyone who has not agreed to use it as follows :
The weapon to be coated, whether metal or otherwise, must have all of the oil poured over it’s entirety while being held by it’s owner. The oil must be allowed to cover both the weapon and the wielder’s bare hand(s). If The Purifier is tricked into opening and used in any other fashion it dies shortly thereafter, taking it’s boons with it.
The weapon thus coated becomes enchanted, capable of striking even insubstantial or magical foes. It glows with a warm blue radiance when drawn and held aloft. Those seeking to harm or deceive the wielder take on a green, sickly pallor under such light. Any non-sentient undead struck with the weapon are dispatched immediately.
The Purifier will make a demand of the wielder, framed thusly :
You must never _(something dishonorable)___
You should give all you do not need to survive to the poor!
You should help anyone who requests your aid.
You should hunt (the undead, demons, devils, agents of chaos) whenever the chance arises!
Or another noble-but-tiring demand of the GM’s choosing.
The character bound to and wielding The Purity may at any time, including after rolling the dice, declare an attack roll made with the weapon to be a twelve. If they do so they become bound to The Purity’s latest demand, forever. It will quickly make another.
If the wielder breaks an oath they have made to The Purity they become wracked with a supernatural illness and begin to waste away, an unnatural starvation, until they take steps to rectify their “transgression.” Whether this can be ended by shattering the bound weapon or whether it is a burden to be borne for their lifetimes is up to the GM.