Death's Door

You grant a dying creature the ethereal drive to push past its mortal limits. As the creature's soul begins leaving its ravaged body, you will the life force back into the creature, albeit by forceful means. The creature's soul becomes physically bound to its body, rather than within it. The soul, however, is not meant to experience physical damage, and as such serves as a metaphorical "glass armor" that has a 50/50 chance of being destroyed upon impact. 

If this spell is cast within the Ethereal Plane, it takes no effect and the spell is wasted.

When a willing creature you can see within 30 ft. drops to 0 hit points, you may cast this spell as a reaction to prevent them from falling unconscious. A creature affected by this spell has the Death's Door status effect for the duration of the spell.

A creature with the Death's Door status effect cannot fall unconscious by being dropped to 0 hit points and does not make death saving throws on its turn. Whenever the creature would normally take damage, the creature instead makes a DC (10+CON Mod) Constitution saving throw. On a success, the creature takes no damage, and on a failure, the creature dies instantly. Each individual damage source (not damage type) incites a separate saving throw. This effect is removed whenever the creature regains at least 1 hit point or the spell ends.

If the spell is still in effect when the target is subjected to an effect that would kill it instantaneously without dealing damage, that effect is instead negated against the target, and this spell ends.

If the spell ends while the creature has 0 hit points, the creature immediately falls unconscious and makes death saving throws on its turn, as though it just dropped to 0 hit points.

"Death will have the last laugh in the end, so why not play a game or two of chance in the meantime?"

-Heinrich Lodestar, "Discoveries of a Corpsemaker"