Deus Ex: Mankind Divided, like previous Deus Ex games, will track the decisions you make and what you do – especially in the final few mission, can have an effect on the different Endings to the game.
The ending to the game largely revolves around how you tackle the final two missions: Mission 16: Stopping Marchenko, and Mission 17: Protecting the Future. The outcome of these two missions can also change depending on if you decided to do either Mission 11: Confronting the Bomb-maker or Mission 12: The Heist.
At the end of Mission 15: Securing the Convention Centre, Marchenko will announce that he plans to blow up the surrounding residential towers unless you find him immediately, and that he will also be poisoning Nathaniel Brown and the delegates with the Orchid. If Mission 15 ends early due to the alarm being sounded, the operatives in the reception area will open fire and start killing civilians. You’ll also find that Director Miller has already been poisoned by the Orchid.
Despite Marchenko’s words, he won’t go about destroying buildings until after around 10 minutes since you start the trek to stop him. This means if you are fast, you can actually save the delegates first and then find him before he even pulls the trigger. If you need more time, getting the bomb diffuser for talking down Allison Stanek the bomb-maker will let you diffuse the bombs remotely, thwarting one of Marchenko’s plans entirely.
Even with the poisoned champagne in the room, Nathaniel Brown talks enough that he and his delegates won’t actually drink the poison until around 10 minutes after making the choice to go save them. Even after that, they’ll only be recently poisoned, and won’t die on the spot, giving you a chance to come to the rescue and deliver an antidote, if you raided the VersaLife bank and found the antidote for the Orchid.
Since Miller is already poisoned, he will die if you do not have the Orchid antidote, which means you’ll have to choose between giving it to him, or saving it for the delegates if you don’t think you will make it in time.
Deus Ex Ending Scene
There is a way to make pretty much everyone happy – and even take down Viktor without killing him if you truly want to save everyone. To do this, you must have gotten the Orchid cure by completing mission 12 and raiding the VersaLife vault.
During Mission 15, make sure you don’t alert any of the disguised operatives in the Reception Hall, to ensure no civilians are killed. Once you reach the catering area, head past the door to the catering room to find a lone guard at a desk, and grab the Security Stash Keycard on the desk near him (you can also find Marchenko’s killswitch under the desk behind some studio bags if you didn’t find it at the G.A.R.M. site).
Enter the catering area and find Miller, talk to him after Marchenko’s speech, and give him the Orchid Antidote. Now immediately run back to the reception area and drop to the first floor, and take the double doors to find Marchenko in the Exhibition Hall. If your idea of a happy ending doesn’t involve Marchenko living, you can kill him immediately using his kill-switch.
If every life is valuable, you can still knock him out fairly quick, as long as you can stun him with a device like an EMP Grenade or Tesla fist augmentation, and then run up to do a non lethal takedown.
With Marchenko down, instead of backtracking all the way to the reception hall, go to the top floor of the Exhibition Hall and look for a bar area on the right near one of the security laptops that has armed mines all around it. Carefully crawl behind the counter and look for a secret button under the countertop, and unlock it with the Security Stash Keycard. This opens up a secret door next to the bar that leads to through the kitchens and straight to Nathaniel Brown in mere seconds!
This can be used whether you reach Brown or Marchenko first, but since the path to the delegates is heavily guarded regardless of any alarms, doing Marchenko first will save a lot of time.
Wondering what effects your other choices had over the course of the game? Watch the ending sequence, and odds are that Eliza Cassan will be talking about many of them in the final news report.