Death's Needle

As a bonus action, you enchant a weapon with anti-life energy.  Any weapon that deals piercing damage is charged with necromantic energy.  For the duration of the spell the weapon is considered magical for the purpose of overcoming resistance, if it is not already a magic weapon.  A successful melee attack is required.  The damage dealt by the weapon is its normal damage but is now necrotic.  

At higher levels the spell does additional necrotic damage,  1d6 at 5th level, 2d6 at 11th level, and 3d6 at 17th level.