In the final level Requiem, Diana will give you the antidote to the death serum. You will fade in and out of life (power bar rises and drops to the beat of Agent 47's heart) while the credits roll. If you do nothing, the game will end. However, if you press the Left Analog-stick Up and Down (or any other buttons) repeatedly, you will come back to life and your mission will be to leave no witnesses.