Regeneration, Variant (5e Feat)
From D&D Wiki
Could be considered a Feat for Character Level 10+）Hit die = Class Hit points ｄ＃＋Constitution Modifier. Your able to use your Hit Die to prevent your HP dropping past 0, and avoiding death saving throws.
Once your HP drops below 1; Instead of Rolling for Death Saves on your next turn. You roll Your Hit Die instead to Add to your HP.
Any excess surplus damage dealt to you that causes your HP to become a negative number (-#); you roll your Hit Die to add to the negative number to make it above 0.
Once the last roll of the Hit Die that causes your HP to become (+#) positive. You can't add another Hit Die to your HP, even if you have some Hit Die left.
You're still unconscious for that Round; you can get up on the next Round． —— OR in ［6－(Constitution Modifier)］ rounds． —— OR in d6－(Constitution Modifier) rounds．
If Massive Damage would normally kill you instantly, and you have Hit Die left and are able to regenerate, you remain alive but unconscious.
If at any point you are at 0 hit points and are unable to regenerate, then you are truly slain. And start Rolling Death Saves on the Next Turn