Alive is a very short stage in A Plague Tale: Innocence. Completing it will take only a few minutes, and it comes down to wandering around Chateau d'Ombrage.
First, ask Roderick to break the door. Proceed through the narrow corridor. It will lead to the next door, which also have to be opened by force. Thus, you will get to the yard. After a short conversation, you will have to follow for Lucas to the main lobby.
After the cutscence comes the end of the task.