Red Lady’s Scalpel

Weapon (dagger), legendary (requires attunement)

This silver surgeon’s scalpel is permanently stained with dried blood, it now spreads pain and disease. The scalpel functions as a +1 dagger of wounding that does slashing damage rather than piercing. Hit points lost to this weapon’s damage can be regained only through a short or long rest, rather than by regeneration, magic, or any other means. Once per turn, when you hit a creature with an attack using this magic weapon, you can wound the target. At the start of each of the wounded creature’s turns, it takes 1d4 necrotic damage for each time you’ve wounded it, and it can then make a DC 15 Constitution saving throw, ending the effect of all such wounds on itself on a success. Alternatively, the wounded creature or an ally within 5 feet of it can use an action to make a DC 15 Wisdom (Medicine) check, ending the effect of such wounds on it on a success.

Once per day, you can cast contagion (save DC 18) as a bonus action on a creature that was just wounded by the blade.

The essence of an ice devil currently resides in the Red Lady’s scalpel. From there it can possess humanoids. While attuned to the dagger, you must make a DC 15 Charisma saving throw every day while in possession of it; each failure moves your alignment one step toward lawful evil. Once your alignment reaches lawful evil, The ice devil takes control of you immediately when you fail a third daily saving throw in a row. Dispel evil and good drives the devil back into the dagger temporarily, but the devil can repossess you again as an action. To drive the devil back to its home plane, dispel evil and good must be directed specifically at the dagger.