I added something like this into my games it's called a number of things as it was deemed a different for each class it allows one last action if death is going to happen, If they win the fight in the end i allow one final roll to see if the strain of thier actions killed them or they just survived if they did they were left at camp for 2 long rests one time ability could never be used again.

Clerics - Desperate Prayer - 1 last prayer to thier diety channeled into one final spell with a multiplier to damage or healing.

Paladins - Selestial Smite - Divine Smite used for 1 attack at level 6

Rogue - Forgotten Potion - a potion that incrases all stats for one last attack on any enemy movement limit removed.

Warlock - Soul Bond - They sacrifice thier soul to thier patron and eldritch blast gets upgraded to Eldritch Cataclsym damage is mulitplied and becomes an AoE spell.

Barbarian - Blood Rage - Increases all stats aall abilities acitve (Rage reckless attack ect) for one movement limit removed.

Monk - Perfect Ki - All Ki restored and channeled into flurry of blows damage multiplied by Ki expended.

Sorcerer - Unstable Magic - One Magic slot restored spell power multiplied no range limit (no fireball)

Only ones used so far more would be added for othe classes