Hey everyone!  As someone who loves creating homebrew magic items for fun, I sometimes have trouble determining the rarity I should ascribe to them.  With that in mind, this is a new, very odd idea of mine that I could use some input on.

Here's the basic concept:

This magic weapon has a +1 to attack and damage rolls, and can come in the form of a longsword, greatsword, shortsword, rapier, or scimitar.

As a bonus action, you can summon this magic weapon back to your hand if it's not on your person.

Before the result of an attack roll is determined, you can subtract up to 20 of your hit points.  If the attack hits, those hit points are converted into Psychic damage to your target.

It requires attunement.

So, what rarity should I give this item, and is it even a good concept?

Edit #1: The weapon now causes Necrotic damage instead of Psychic.  You take Necrotic damage instead of simply subtracting hit points, and you also gain a level of exhaustion when you use this feature.  The weapon also no longer has a summoning property.

I would start at Rare and adjust it upwards if necessary to be determined through playtesting.

I'd consider this to be a Very Rare item or a Legendary item.

Flame tongue adds on average 7 points of fire damage to each attack. frost brand adds an average 3.5 damage to an attack. The Devouring Sword can add a whopping 20 points of damage on demand! That's better than making two additional attacks that would each deal 1d10+5 damage, or hitting an enemy with 4th level magic missile spell.

This is an interesting item, not least because of how it changes over levels. At level 4, 20hp would be unthinkable unless going for a finishing blow. At level 15, it's usually going to be worth going for the maxed out extra damage every attack against a powerful enemy creature (a boss, essentially). A PC who upcasts a healing spirit can mitigate a lot of the downside.

Thanks for the in-depth analysis!  I'm also not entirely sold the damage type.  Do you think Psychic works for this type of health draining weapon, or would Necrotic fit better?

Necrotic is probably better for it, because it is siphoning the life of everyone.

Yeah, it’ll pro’ly land around Very Rare. If you could wait until a confirmed hit before spending the HP then I would say Legendary.

Adding a level of exhaustion on use would prevent spamming while remaining in-theme, I think. Also something about how if this damage reduces you to 0 hp you are instantly devoured by the sword. Make the damage necrotic and I think it could be at the higher end of Rare/lower end of Very Rare.

Quote from scatterbraind >>

Adding a level of exhaustion on use would prevent spamming while remaining in-theme, I think. Also something about how if this damage reduces you to 0 hp you are instantly devoured by the sword. Make the damage necrotic and I think it could be at the higher end of Rare/lower end of Very Rare.

 This. I saw Sanvael's post and I thought, "Wow, that would be way too spammable...." Exhaustion is a perfect solution, though maybe you only get it if you transfer more than 10 of your HP to extra necrotic damage. Otherwise, who's gonna only do like 5 damage? The thing about instant consumption by the sword if it takes you down to 0 is good, and thematic, too.

Very minor thing: I would also change the wording to say that you take necrotic damage that can't be reduced in any way (instead of saying you reduce your current hit points, since official stuff says it like "you take damage," not "you reduce your current HP"), and if the attack hits, the target takes additional necrotic damage equal to the damage you took.

Oh, I like this; I like this a lot!