[Infernal] Weapon - Soulsnare

The sword originally looked more like a rune-etched blade, but over the course of design I added the mouth and really wanted to try a split-blade look, which led to its current magma-filled interior. The sword also leans on the devil's contract theme, drawing most of its power from the summoning aspect.

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The blade of the sword opens up in a fiery maw, spewing with hellfire that binds a slain creature to its service. Despite appearances, the specters answer not to the wielder but to the entity trapped within the weapon, their spirits broken by the violent fate that befelled them.

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Soulsnare
Weapon (longsword), legendary (requires attunement)

Once attuned to this sword, your alignment drifts one step toward lawful evil every 2d4 days. You also cannot voluntarily end your attunement to the sword. If you are targeted by a spell that ends a curse, you end your attunement.

Sentience. Soulsnare is a sentient lawful evil weapon with an Intelligence of 15, a Wisdom of 15, and a Charisma of 16. It has hearing and darkvision out to a range of 120 feet unimpeded by magical darkness. The sword communicates telepathically with its wielder and can read, speak, and understand Infernal.

An ambitious devil named Zaelith lives inside longsword. It had been trapped there by an archdevil centuries ago for a debt for which the devil must reclaim souls to pay off. It mechanically plans for the corruption of good minds and revels when the work is done. If the devil grows dissatisfied, it can take control of any specters resurrected by the sword for its purposes.

Joined Spirit. While you hold the sword, you have the following benefits: 

• You can read, speak, and write Infernal. 
• You gain darkvision out to 120 feet if you don't already have it. 
• You have advantage on Charisma saving throws.

Spur of the Cinderdeep. You have a +2 bonus to attack and damage rolls made with the sword and you deal an extra 1d6 fire damage to targets you with it. When you kill a humanoid with this sword, you can use your reaction to snare its soul and resurrect it as a specter. This specter obeys your commands and takes its turns immediately after yours. If you don't give it a command, it takes the Dodge action. The specter remains for 24 hours, or until it dies or you resurrect another specter. When it dies, the specter's soul is sent to the Cinderdeep instead of its expected afterlife.

Crush Soul. While holding this sword, when a creature within 60 feet of you succeeds on a saving throw, you can use your reaction to force the creature to reroll the d20 and use the new result. Once you have transformed a successful save into a failure, you can't use this property again until the next dawn, unless you reduce your specter to 0 hit points to do so.