This page of the guide will show you how to unlock the secret ending of Hitman 3, while completing the Untouchable mission on the Carpathian Mountains map (Romania).
In addition to the main ending of Hitman 3, players can unlock another secret ending to the third installment of Agent 47 adventures, which is related to the A New Father challenge. Below you'll find a description how to do this.
After meeting Arthur Edwards (The Constant), pick up the serum and wait approximately one minute. You don't have to do any extra activities, you can just stand still. It is important to keep the syringe in your hand.
After some time, Arthur Edwards will speak up and begin encouraging Agent 47 to take a dose of the serum. An additional option to use the syringe will be unlocked on the screen (LMB on PC). Injecting Agent 47 with the serum activates the secret ending of Hitman 3.