Spire of the Scorned

Statistics
Weapon (Rapier(Saber)) legendary (requires attunement)

You gain a +1 bonus to Attack and Damage Rolls made with this Magic Weapon.

When you are attacking a creature with the Fiend (Demon) Subtype with this weapon, you instead gain a +3 to Attack and Damage Rolls.

Once per day you can cast the spell Invisibility, targeting only yourself. This ability recharges on a long rest.

This weapon ignores resistance to slashing damage, and is of quality equivalent to adamantine.

After scoring a critical hit, and after rolling damage, you can choose to decapitate the creature if it has a head. The creature dies if it can't survive without the lost head. A creature is immune to this effect if it is immune to slashing damage, doesn't have or need a head, or if the creature is Huge or larger. Such a creature instead takes an extra 5d6 slashing damage from the hit.

Curse of the Merciless
When you kill a creature by decapitating it with this weapon, make a DC 13 Constitution saving throw. On a failed save, you receive half of the creature’s remaining health as necrotic damage, or a quarter of the creature’s remaining health on a successful save.