Deep Magic Volume 2 (p.154)
Compel Mercy
1-level Enchantment
Casting Time: 1 reaction, which you take when a creature you can see within 30 feet of you hits another creature with a weapon attack
Range/Area: 30ft.
Components: Verbal, Somatic
Duration: Instant
Attack/Save: WIS Save
You force the attacking target to be merciful. The target must succeed on a Wisdom saving throw or the damage from the triggering attack is halved. If the damage would still reduce the attacker’s target to 0 hit points, that creature is knocked but stable instead.
Available for: Cleric, Paladin