Based on similar feat by goldenb0ss

If you would go unconscious from damage, instead you roll a Constitution Save. If you succeed, you are at Death's Door until you are healed or killed.

While at Death's Door:

> you can only take 1 action, 1 bonus action or 1 movement action on a turn and you can no longer concentrate on spells
> your movement speed is reduced to 5 ft
> you roll death saves as normal and death saves rules apply as normal.
>  if stabilized, you are no longer on Death's Door and regain 1 hit point.