Abyssal smite

The target takes an extra 1d10 Necrotic damage from the attack as shadowy tendrils sprout from the damage dealing part of the weapon and engulf the target. The target is grappled for the duration or until it escapes (DC is your spell save DC). While grappled, the target's speed is 0, and it takes 1d12 Bludgeoning damage at the start of each of your turns. Attacks made with the same weapon as the one used in the casting of this spell deal made against the target deal an extra 1d8 Necrotic damage.

Using a Higher-Level Spell Slot. The initial Necrotic damage increases by 1d10 for each spell slot level above 1.