Back at St. George's, read the note Malia left on the desk. Mosely will appear from your room. Go back there to talk to him. Talk to Mosely. You can give him his wallet back by choosing "Credit Card" if you want, but it doesn't make a difference. Now choose "Make Plans". Gabriel will give Mosely the Tracking Device. Read the newspaper, then head to the Cathedral.
Go into the confessional booth and use the Snake Rod on the knothole. Once the elevator stops, place both the Snake Rod and the Signaler under the kneeler for Mosely.
If you are wanting to get the Solo Mission achievement, do not place these items. You can still finish the game this way, but you will not get full points.
Now, step out into the foyer and save your game. There are some Game Over situations ahead. Now go through the door on the other side of the room. The Hounfour will be set up just like the Snake Mound. Go up to go clockwise and to access the various rooms. First, go up once and enter Room 7. Take 2 robes and both the boar and wolf masks. Most of the following rooms are locked or empty.
Keep circling clockwise until you reach Room 2. Enter the room and watch the scene. Afterward, exit and go up to Room 3 and enter. This is Dr. John's... uh... room? There is a key card on the wall but don't take it, unless you want to watch Gabriel die. Simply exit the room without doing anything else.
Go up again and enter Room 4. Take the Black Book sitting on the bottom right desk. Exit, then go up once more and enter the archway on the right side. Enter the main room and use the black book on the drums. Select "Summon" and "Brother Eagle". Now you will need to move quickly, as this will call Dr. John to this room. You do not want to cross his path either.
As soon as Gabriel is done playing, go through the top right path, then go up to Room 3. Take the keycard and immediately go up again. You should hear Dr. John return to his room and shouldn't have to worry about him after that.
We can now check out those locked rooms. We want to go to Room 8 last, so circle down back to Room 1 first. This is my favorite room in the game. Take the money three times (until Gabriel says he can't carry anymore) and leave. Continue down to Room 11, and again use the keycard to enter. Have a look around, then leave once you are done.
This is your last chance to save your game; you will not be able to again once you go inside Room 8.
When you are ready, continue down the hallway to Room 8, again using the keycard to go into the room. Use the Talisman on Grace, then open your inventory to combine the masks and robes. Give the Boar Disguise to Mosely and watch the cut scene.
When you regain control, use the talisman on Tetelo. Watch another cut scene, and when that one is over, throw the talisman to Mosely. Tetelo will then throw you to the ground. Grab the idol to have Gabriel smash it. After yet another cut scene, you will have a choice of how you want to end the game. You can use the hand icon on Malia to try to save her and get the good ending. Otherwise, you can use the dagger on her and receive the not so good ending.