I decided to stick around and steal his house blind before returning the diary to Armand, but now Amantius woke up and the door to his house is locked, with the magical requires a key lock.

Any tips? Console command or whatnot? His cellar is filled with blood but I did not see an entrance to the sewers there.

EDIT: Ok I missed the actual door and tried one that was not leading out of his house, I wonder where it leads though.

Sorry for bumping other topics down.

The door at the bottom of the stairs? It doesn't actually go anywhere. Could be something left over from a portion of this quest that was cut, there is dialogue left over in game that refers to a different sequence of events.

There is a door to the sewers in the basemant though.

Did Alectus catch you? If not, pickpocket his key from him, unless it is the door Rhomphaia mentioned. That one has no key and doesn't go anywhere.