Nine Lives Stealer

Weapon (any sword), very rare (requires attunement)

You gain a +2 bonus to attack and damage rolls made with this magic weapon.

The sword has 1d8 + 1 charges. If you score a critical hit against a creature that has fewer than 100 hit points, it must succeed on a DC 15 Constitution saving throw or be slain instantly as the sword tears its life force from its body (a construct or an undead is immune). The sword loses 1 charge if the creature is slain. When the sword has no charges remaining, it loses this property.

In these lists

Very rare magic items
Magic weapons
Weapons table

Questions

Does the Nine Lives Stealer require attunement?

Yes: it requires attunement.

How rare is the Nine Lives Stealer?

It is a very rare weapon (any sword).