Blade of Ker

Black, feathery wisps of darkness cover the weapon used in casting. Until this spell ends, any attack you make with this weapon deals an extra 4d10  Necrotic damage with the attack. The damage increases by 1d10 if the target has half their hit points or fewer.

Using a Higher-Level Spell Slot. The damage increases by 1d10 for each spell slot level above 6.