Dire Death (5e Variant Rule)
From D&D Wiki
This is meant to change how PC's die, both in and out of combat.
If a PC's HP reaches 0 they become unconscious. Player characters may have negative HP. If their HP ever equals, or exceeds their maximum negative HP, they die. (Ex. If a PC with 20 max HP, reaches -20 HP, or lower, then they die.)
Most forms of damage should cause a PC to slowly keep taking damage after they reach 0 HP. Be they bleeding or poisoned, etc. Once they are reach 0 HP, each turn they must succeed a DC 10 + their current negative HP Constitution saving throw. They take 1d4 damage on a failed save, and no damage on a successful one. Ex. If they have -4 HP, the DC is 14.
If they roll critical failure (1), they take 2d4 and if they get a critical success (20), they stabilize, and stop taking damage. They can use any number of your hit dice to heal.
A creature can heal a PC with healing magic, but they must be healed out of the negatives and up to 0 to be stabilized or healed to 1 to wake up.
Stabilized means they no longer have to make Constitution saves and no longer take damage.)
A creature may also stabilize them with a medical kit and a medical roll. The DC for this is equal to that of DC they would make on their turn. Ex. If they have -4 hp the DC is 14. If you don't have a medical kit that DC increases by 5. If you roll a critical success (20) on your medical check, the downed player may use any number of their hit dice to heal themselves.
There are new rules for falling damage. Fall damage is equal to 1d10 damage for every 10 fallen past the first 10, rounded up. Ex. If a player falls 6 feet, you round it up to 10 for 0d10. If they fall 35 feet, you round up to 40 for 3 d10.
Not Breathing: A PC can hold their breath for a number of turns equal to 10 + 5 times their Constitution modifier. If a PC isn't given a proper amount of time to hold their breath, then they can hold their breath for a number of turns equal to their Constitution modifier.
Every turn after the point where they could hold their breath, they receive 10 damage, and every other turn they take a level of exhaustion.