Death Defiant

The first beckonings of the beyond beginning to whisper in your ear, you summon forth every ounce of will you possess, and say no.

• Your Constitution score increases by 1, up to a maximum of 20.
• When you drop to 0 hit points, instead of falling unconscious, you, instead, gain the condition, "Last Stand". While affected by Last Stand, you only have 1/2 your normal move speed. You also have disadvantage on attack rolls, saving throws, and ability checks, while enemies will have advantage on saving throws against you. If you have any levels of exhaustion, you're movement drops to 1/4 it's normal speed instead, and your rolls with disadvantage get a - 1 modifier for each level.
• Your death saves, instead of being straight rolls, become Constitution saving throws of DC 15 - CON. Rolling three successes or a natural 20 restores you to 1 hit point, and removes the effects of Last Stand from your character, while three failures means your character dies.
• Stabilizing effects restore you to 1 hit point.
• Damage against you, while in death saves, instead of counting as auto-fails, force you to roll another death save
• If your Constitution score is at least 18, natural ones only count as one failure, over of the two they count for normally.

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The Cost to Endure: Fighting to stay alive past the point where your body should've faded may give you a last chance in the face of demise, but it comes at a price. For every 3 successful death saves, through the first 9, your character counts as having one fail whenever they're at 0 hit points (3 maximum). If your character gains 3 failures in this way, instead of immediately dying, they keep rolling death saves when they need to. However, while they still need 3 successes to get out of them, only one failure is needed to mean they die. Fails from The Cost to Endure and total successes reset, after they take a long rest.

In the Wake of Carnage: The inferno is quelled, and it's roar quiets. What flames remain quickly flicker out in the cold torrent of the after, and with them, so do the fires sustaining you, as well. At the end of a combat where you rolled death saves, roll one more Constitution saving throw of DC 10 + the number of saves you made/2. On success, you gain one point of exhaustion for each Cost to Endure fail, you have, and on a fail, you fall unconscious for 1d4 + 2 hours. If you had the maximum amount of Cost to Endure fails, on a success, you instead fall unconscious for 1d6 + 2 hours, while on a fail, you die. If you fall unconscious, you gain the effects of a short rest, when you wake up.

This feat isn't necessarily meant to be a buff to characters in death saves, but is instead meant to provide them an alternative option to the standard ruleset, that while potentially more punishing than it, hopefully offers players more choices and expression with their death saves.