Wraithblade

You summon a blade made of dark energy in your hand. This blade hungers for death and guides your strikes. Make a melee spell attack with the weapon in which a critical hit is 18-20. On hit do 1d8 necrotic damage. If the hit is critical, roll 3 times the dice instead of 2. The blade dissipates at the end of the strike.  

This spell's damage increases by 1d8 at 5th level (2d8), 11th level (3d8), and 17th level (4d8).