
Adventurers often disregard grievous injury to continue fighting at the expense of their future well-being. You gain the following benefits.
Increase your Constitution score by 1, to a maximum of 20.
If you would fall to 0 hit points or die, instead, you may permanently mark a death saving throw as failed and your hit points become equal to 1/3 your total health rounded up. If all death saving throws are marked as failed then your character dies and cannot be resurrected outside of divine intervention.
If a death save is marked as failed by this ability then you require that many less successful death saving throws in order to stabilize.
"I can't die. I'm too busy for that." - Rawlins, Captain of the Watch

I did not put a level prerequisite on this because the pc only has two revives as a nonrenewable resource (at dm's discretion). Taking this feat earlier on and using the revives actually makes gameplay more difficult because, in most cases, a lower level pc has a lower chance of making the death saves compared to a higher level pc and reducing the number of saves they can fail puts the rest of the party on a timer to heal them.
I really like this idea, although i think losing the death save forever is kinda too much, maybe some high level spell would be able to fix it like greater restoration at 7th or 8th level make it, then again you can get these spell slots back after a long rest so who knows, still think there should be some kind of way to get rid of it
I think this is a great feat. As far as removing the permanent death save, the DM always has final say. If one DM wants to keep it as-is, cool. If another DM sees this as a great plot for a quest, cool!
Just to respond to your idea, I LOVE the idea of using this as background for NPCs. Your NPC gets to be a OP powerhouse that can save the party from one bad situation, but because of the choices in their past, they don't get to recover like other people. When they go down, that's IT.
I think this needs a bit of a rewrite, it's a little confusing.
0 hit points OR die?
Maybe permanently mark a death saving throw as failed.... You may permanently unmark one death saving throw after completing a long rest (or after rolling a 19 or 20 Con Check on a Long Rest)
This is the Gambler's blade: Hardcore edition
I think it’s either: you can mark it as soon as you would drop to 0 or instantly die and if you don’t you can’t use it but you might be saved or you can use right when you drop to 0 or when you would later on die from failing your death saves and the way it’s worded would be up to interpretation from the dm
Imagine getting absolutely glassed by a dragon and just saying: "Light work no reaction"
It would be neat if only a spell such as the Wish spell could undo the permanent fail, or divine intervention (as the description says). This would make it possible to recover, but also very difficult.
Another idea would be somehow only gaining disadvantage to death saves temporarily, and limiting the ability a little.