Death by a Thousand Cuts (HB)

You unleash a barrage of slashes. Choose a creature within range, and teleport to an unoccupied space you can see within 5 feet of it.

Make 9 melee spell attacks against it, each attack dealing 1d6 slashing damage. If 5 or more attacks hit, deal an additional 30 slashing damage to the target. If 7 or more attacks hit, the taget is wounded, and for the duration, it has its speed halved and it has disadvantage on all attacks and saving throws using strength or dexterity. At the end of each of its turns, a wounded creature can make a constitution saving throw to stop the bleeding, taking 2d6 necrotic damage on a failed save, and ending all negative effects on a success. A critical hit counts as 2 succesful attacks for the purposes of these effects. 

Once you have made all attacks with this spell, you may choose to teleport up to 60 feet away, to an unoccupied space, you can see within range.

If you hit with 15 attacks, the taget dies instantly.

At higher levels. When you cast this spell using a spell slot of 7th level or higher, the number of attacks increases by 2 for each slot level above 6th.