Bleeding (5e Variant Rule)
From D&D Wiki
You’re standing in a street. When suddenly, a man walks up to you with a dagger in hand and stabs you in the chest six times. On the fifth, your DM says, “It’s a critical hit, start bleeding.”
How it works
Whenever someone or something makes a critical hit on a target, dealing slashing or piercing damage, if the target is has blood, or some other similar body fluid, the target must make a DC 15 Constitution saving throw, on a fail, they start to bleed. At the end of every round while the target is bleeding, they take 2d4-1 necrotic damage. For every point of damage that the target takes because if the bleeding effect, they gain a a stack of “wounds.” This effect ends when the target takes 30 necrotic damage from the bleeding effect, where they will fall asleep for 2 hours, when they wake up, they will have 1 hit point. Alternatively, the target can make a Wisdom (medicine) at the end of their turn, before they take their damage from the bleed effect. This check has a DC of the amount of “wound” stacks the target has, and on a success, the bleeding effect ends.