Staggering Smite

Source: Player's Handbook

Level 4 Enchantment (Paladin)

Casting Time: Bonus Action, which you take immediately after hitting a creature with a Melee weapon or an Unarmed Strike
Range: Self
Components: V
Duration: Instantaneous

The target takes an extra 4d6 Psychic damage from the attack, and the target must succeed on a Wisdom saving throw or have the Stunned condition until the end of your next turn.

Using a Higher-Level Spell Slot. The extra damage increases by 1d6 for each spell slot level above 4.