Tales of the Bloodstone Lands
Slight Change to Rules as Written here.
If your intent is not to kill, but to incapacitate or knock out an enemy, you must make that determination before you attack. You can only make non-lethal attacks when attacking a single target. Area effect attacks cannot be controlled to that extent. “Yeah, I centered the fireball on him, but I only meant to knock him out.”
A target who has been reduced to 0 hit points using non-lethal damage is considered to have advantage on Death Saving Throws, even if a mix of lethal and non-lethal attacks were used on the target. Most targets stabilize in such a case, only the extremely unlucky will pass.
Note: This applies to PCs targeted with non-lethal attacks as well.
Rules as Written
Sometimes an attacker wants to incapacitate a foe, rather than deal a killing blow. When an attacker reduces a creature to 0 hit points with a melee attack, the attacker can knock the creature out. The attacker can make this choice the instant the damage is dealt. The creature falls unconscious and is stable.