Bashael's Lingering Servitude

This arm is the last remnant of the devil Bashael, trapped in Ravenloft by Strahd's magic. When attuned to this weapon, your Charisma score increases by 2, as does your maximum for that score. Additionally, it will always attempt to bond with the person attuning by replacing their left arm. Functionally, an attuned user can use this limb usually as though it were an arm.

Attunement to this weapon allows the attuned creature to take their action and use the Spellcasting feature to expend two charges to cast first level False Life, raising the spell for every charge past the first two expended up to the fifth level, expend three charges to cast second level Alter Self (Change Appearance or Natural Weapons only), or expend four charges to cast third level Fireball, raising the spell level for every charge past the first four expended up to the fifth level.

Innate Spellcasting. Bashael is an innate spellcaster and can cast Produce Flame and Charm Person at will.

Spellcasting. Attunement to Bashael's Lingering Servitude allows the user to access Bashael's spellcasting, allowing them to cast the following.

At will: produce flame, charm person
1st level (2 charges per cast): false life
2nd level (3 charges per cast): alter self
3rd level (4 charges per cast): fireball

Fiendish Endurance.  If the weapon has at least one expended charge, the user can use their action to begin a ritual to recharge missing charges. Once per long rest, the attuned creature can take a minute uninterrupted by any other action to channel energy and restore 1d6 charges to the weapon. Doing such requires them to make a DC 22 Charisma check detailed under Curse.

Sentience. Bashael's Lingering Servitude is a sentient lawful evil weapon with an Intelligence of 13, a Wisdom of 16, and a Charisma of 22. It has hearing and darkvision out to a range of 120 ft.
The weapon can speak, read, and understand Infernal and Common, and can communicate with its wielder telepathically. Its voice is shrill and brittle. While you are attuned to it, Bashael can also perceive the world through your senses.

Curse. Binding yourself to Bashael's Lingering Servitude has certain risks. When you first attune to the weapon you become cursed and are no longer able to unequip the weapon. After attuning to the weapon, reaching 0 charges, or taking a turn to regain charges using the Fiendish Endurance feature, make a DC 22 Charisma save. Make the save with advantage if you know Bashael's true name. On a failed save, your alignment changes to lawful evil and you take one level of exhaustion and are charmed by Bashael for 3d8 hours. During this time, you feel obligated to obey any order or request given by the devil and cannot change your alignment. If you pass the save, your alignment remains the same and you have advantage against the save for the next week.
Failing this save twice in a row or five times over the course of a month spreads Bashael's corruption across your body. He refuses to elaborate on what this means.

Notes: Bonus: Charisma Score, Bonus: Ability Score Maximum, Expertise: Persuasion, creature with a pact to Bashael, Buff, Cursed, Handwear, Sentient