Shadow Strike

5th-level Transmutation
Casting Time: 1 bonus action
Range: Self
Components: V
Duration: 1 minute

You charge a weapon with shadow energy. The weapon ripples with anticipation of slaughter and yearns to get out. The first time you hit a creature with a weapon attack before this spell ends, the shadowy energy lashes out, dealing an extra 6d12 shadow damage to the target. The spell ends early if you cast it again or cast another smite spell, such as shackling smite or lightning arrow.
If you make a weapon attack against a target in dim light or darkness while under the effects of this spell, that weapon attack is made with advantage and inflicts a critical hit on a 19 or 20.
At Higher Levels. When you cast this spell using a spell slot of 6th level or higher, the damage increases by 1d12 for each slot level above 5th.

Source: Homebrew