Blackrazor 5e

Weapon (greatsword), legendary (requires attunement by a creature of non-lawful alignment)

Hidden in the dungeon of White Plume Mountain, Blackrazor shines like a piece of night sky filled with stars. Its black scabbard is decorated with pieces of cut obsidian.

You gain a +3 bonus to attack and damage rolls made with this magic weapon. It has the following additional properties.

Devour Soul. Whenever you use it to reduce a creature to 0 hit points, the sword slays the creature and devours its soul, unless it is a construct or an undead. A creature whose soul has been devoured by Blackrazor can be restored to life only by a Wish spell.
When it devours a soul, Blackrazor grants you temporary hit points equal to the slain creature's hit point maximum. These hit points fade after 24 hours. As long as these temporary hit points last and you keep Blackrazor in hand, you have advantage on attack rolls, saving throws, and ability checks.
If you hit an undead with this weapon, you take 1d10 necrotic damage and the target regains 1d10 hit points. If this necrotic damage reduces you to 0 hit points, Blackrazor devours your soul.
Soul Hunter. While you hold the weapon. you are aware of the presence of Tiny or larger creatures within 60 feet of you that aren't constructs or undead. You also can't be charmed or frightened.
Blackrazor can cast the Haste spell on you once per day. It decides when to cast the spell and maintains concentration on it so that you don't have to.
Sentience. Blackrazor is a sentient chaotic neutral weapon with an Intelligence of 17, a Wisdom of 10, and a Charisma of 19. It has hearing and darkvision out to a range of 120 feet.
The weapon can speak, read, and understand Common, and can communicate with its wielder telepathically. Its voice is deep and echoing. While you are attuned to it, Blackrazor also understands every language you know.
Personality. Blackrazor speaks with an imperious tone, as though accustomed to being obeyed. The sword's purpose is to consume souls. It doesn't care whose souls it eats, including the wielder's. The sword believes that all matter and energy sprang from a void of negative energy and will one day return to it. Blackrazor is meant to hurry that process along.
Despite its nihilism, Blackrazor feels a strange kinship to Wave and Whelm, two other weapons locked away under White Plume Mountain. It wants the three weapons to be united again and wielded together in combat, even though it violently disagrees with Whelm and finds Wave tedious.
Blackrazor's hunger for souls must be regularly fed. If the sword goes three days or more without consuming a soul, a conflict between it and its wielder occurs at the next sunset.

Commentary by Sam West, Twitter: @CrierKobold

Blackrazor is one of the most busted magic swords you can get your hands on. You definitely want to be the one landing the finishing blows to satiate this monstrosity, but if you can feed it, you’re going to feel invincible.

Sentient magic weapons aren’t particularly fleshed out in 5th Edition’s rules; most of their guiding mechanics are baked into their descriptions.
If a creature that would normally get death saving throws is dropped to 0 by Blackrazor, it dies outright instead.
What the “conflict” is for not feeding it isn’t well established, nor are there any rules written here for it, so your DM will largely be the determining factor as to what this conflict looks like.
Blackrazor doesn’t get a turn, but can be your eyes and ears, especially if you are blinded or deafened, or you don’t have 120 ft. darkvision in dark places.
Sentient weapons aren’t creatures, and thus can’t be affected by spells like Dominate Monster or Feeblemind.
Blackrazor casting Haste happens instantaneously, and outside of normal initiative; it can be on another creature’s turn, your turn, in between actions or attacks made on a turn, or any other time the DM wants.

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