Vecna's Razor

Summon a dark spectral blade radiating necrotic energy, and which has the finesse property. This weapon deals 4d8+ Int modifier necrotic damage. Additionally, when you hit a target with this weapon, you can choose to sacrifice your own hit dice to increase damage by the same amount (up to 5 hit dice), using your own life force to increase the weapons lethality. This ability can only be used once per casting. 

Casting this spell at 7th level increases the damage dealt by 1d8, and casting at 8th level increases the damage by an additional 1d8. 

Casting this spell at 9th level however gives additional properties. The casting time increases to 10 minutes, but the spectral blade takes on a more physical form. This removes the concentration restriction, you are able to use the sacrificial ability up to 3 times, the number of hit dice you can choose to sacrifice increases to 8, and the blade lasts until your next long rest.