When Alexander locks you in the cell and tells you to stay there and be killed by the shadow, don't escape. If you simply sit there the red shadow stuff will enter the cell and kill you. You will hear the same words from Alexander as in the Bad Ending when you let him enter the portal.
Bad Ending
Wait for Alexander to open and enter the portal.
Good Ending
Wait for Alexander to open up the portal. Once it opens place Agrippa's head into it.
Invisibility
If you hold a drawer from a dresser in front of you, the monsters won't see you. You can walk right up to them. Once you remove the drawer they can see you again.
Revenge Ending
While Alexander is waiting for the portal to open push over the three pillars that are giving it energy.
Super_Secret.rar Password
There is a rar file named "Super_Secret" in the "redist" folder of amnesia that has a password. In order to view the files inside you need a combination of the 3 endings. Each ending gives you a different part of the password, and when put together you can view the files inside. Below are the pieces you receive from each ending, and the completed password. Revenge ending: lke271 Bad ending: tyr299 Good ending:odn314 Combining them gives you the password of: lke271tyr299odn314.