Merciful Casting

+1 to either Intelegence, Wisdom or Charisma (Maximum of 20)

When you cast a spell that deals damage that takes their Hp to zero or below, you may choose have them downed and stabilized, keeping them alive but unconscious.

If you have the Sculpt Spell class feature this extends to Area of Effect spells, and you can select a number of targets equal to 1 + the spells level to take the damage as if it was from the knock out action. This pool of targets is the same pool that Sculp Spell uses to avoid damaging specific targets (ex: You are casting a level 3 fireball, you can have 3 targets avoid the damage and 1 take damage as though from the Knockout action.)

Note that the Damage type does not change, and may still result in permanent physical effects as per the DM's discretion. Fire spells may still cause permanent burns, even if the spell is made to be nonlethal. It also does not prevent ongoing damage as applicable, which each turn will give them a failed death saving throw until the ongoing damage has ended.

Also to note: Even when holding back your spell the rules of Massive damage still apply. If the damage of the spell would bring them to their HP Max below zero hp they will die of massive damage as normal. it also does not account for other spell effects. as an example: if you cast Disintegrate, the target will still reduced to ash when reduced to zero hp.