Death strike

Fatall wounds wont stop you immidietly. If you are reduced to 0 HP but not killed outright, you can make a attack action (mellee, ranged,spell etc.). You make attack rolls with advantage and saves your opponents make are at disadvantage (example: fireball). This ability cannot be used if you are incapsitated, stunned, petrified or if you were Unconscious prior.

your CON inceases by 1. As does its maximum. 

The first death-save you make in a fight is made with advantage.