In Amnesia: The Dark Descent there are three different endings and each one is triggered by a specific course of action while being in the Orb Chamber.
Daniel's Ending
For this ending, Daniel must topple all the pillars in order to ruin Alexander's ritual. The ground begins to shake and the Shadow surrounds Alexander, who will vanish in a blue mist, screaming. Daniel survives apparently unscathed by the Shadow and will walk back all the way through the castle while saying:
"It was my greatest triumph... and I never looked back. You think I was afraid fleeing Brennenburg? Quite the contrary. I knew it was my purgatory - hellfire made to wash my sins. There's no denying the things I've done. But I have paid my tribute. I gave them that awful man... I did the right thing."
Then Daniel exits the gate in the Entrance Hall with the noise of crumbling stone.
Alexander's Ending
Doing nothing in the Orb Chamber will trigger this ending. After Alexander disappears through the portal, Daniel will hear the prisoners and the little girl he killed begging for mercy while the Shadow invades the room and slowly consumes him. After the screen blackens and the Shadow's roar fades, Alexander's voice will be heard saying:
"Thank you my friend. Your sacrifice won't be forgotten. You'll be celebrated... forever."
This ending can also be triggered while in the Cells. After being locked inside, doing nothing to escape will draw the Shadow to Daniel, who will be killed by it in the same way as in the Orb Chamber.
Agrippa's Ending
To trigger this ending, Daniel must put Agrippa's head in the portal right before the preparation is complete. This will cause Alexander to vanish in the blue mist and the Shadow to attack Daniel who will fall on the floor, screaming. The screen blackens and then floating blue lights will appear on the distance. While Daniel walks towards the lights, Agrippa says:
"There he is. Do you see him Weyer? He deserves so much more. Please help him, I know you can. Don't worry Daniel. It will be alright."