Divine Retribution

1st-level evocation
Casting time: 1 reaction, which you take when a weapon attack reduces you to 0 hit points
Range: Self
Target:
Components: V
Duration: Instantaneous
Classes: Cleric, Paladin
Subclasses:

With a last word before you fall unconscious, this spell inflicts radiant damage to your attacker equal to 1d8 + the amount of damage you took from the triggering attack. If the attacker is a fiend or undead, it takes an extra 1d8 radiant damage. The creature then must succeed on a Constitution saving throw or become stunned for 1 minute. At the end of each of its turns, the creature can make another Constitution saving throw. On a successful save, it is no longer stunned.

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, the damage increases by 1d8 for each slot level above 1st.