Dagger of Faust (5e Equipment)
From D&D Wiki
Weapon (dagger), Legendary (requires attunement)
A small black dagger made from ebony with a skull carved into the pommel. In combat it can be used as a regular dagger or the wielder can make an offering to Faust, the demon within the blade.
As a bonus action, the wielder may take 1d8 slashing damage to summon an object using their own blood. This summoned weapon deals psychic damage equal to the weapon's regular damage plus the amount of damage the wielder took in the summoning process. These summoned objects last for 1 minute.
Faust's lust for blood and power requires that the user gather blood for him daily, in particular he requests 2d8 (this amount is rolled by the DM) worth daily and if denied this, at the next dawn Faust will take direct control of the wielder on a failed DC 15 Wisdom saving throw. Once Faust has gained control over you, he will gather as much blood as he can from any creatures in his immediate surroundings regardless of their relation to the wielder.
Faust greatly desires to not be found out until he has attained his full power and will attempt to hold the wielder to secrecy about his existence within the blade.
Conflict will arise if the wielder either refuses to gather the necessary blood or attempts to alert the party to the true nature of his being.