Start the hub in the Cathedral, facing a sign. Read it. Apparently, Tyranith has struck out against Eidolon by himself. Perhaps you can catch up with him and offer your aid.

Turn left and follow the hall around past the barred door and down some steps to a storage room. Smash one of the crates in the back to reveal a crawlspace. Crawl through (by crouching) and drop down twice to some bars over water. Step on the 2nd bar to break it and drop down into the water.

Turn around and notice a coffin atop a couple of steps. Press the camouflaged button on the coffin, opening a tunnel to the right. Hop up and crawl through, dropping back down into the water.

There will be a door on the right and a white teleporter on the left. Open the door, then go through the teleporter to Tyranith's bedroom. Read the book, which says, "They have desecrated this holy place. For that I will never forgive them. Eidolon must be behind this! Only his foulness could penetrate this far into our realm. He will pay!"

Turn around and go to the foreroom. Notice the pool of holy water on the left (you will need to return here later). Go through the door, and follow the hall to the right, back to where you started the hub.

Turn left and go down the narrow hall to the spiral wooden staircase. Go up the stairs past the 1st door to the 1st doorway on the left. Come to a courtyard.

Climb up the rampart to a room. Notice the note on the bench saying that only 2 golden lions stand between you and the dark mage. Break out the golden lion tapestry in front of you to reveal a secret passage.

Take the secret passage to the lift. Take it up and go through the door to the Dark Mage's Laboratory. Defeat him and grab the sheet music from the table (apparently the Dark Mage is an Andrew Loyd Weber fan).

Retrace your steps to the door leading to the Cathedral. Enter it.

Go left up the stairs and come to a room with stained glass on the right and an organ at the end. Approach the organ to play the sheet music. This opens a doorway out on the other side of the stained glass. Break it out and follow the corridor up a ramp and into the bell tower. Drop down beneath the bell to the room with the body of Tyranith. Creep through the teleporter and find yourself on a ledge overlooking a cathedral altar.

Grab the Defiled Holy Relic next to you on the ledge. Drop down and go through the teleporter behind you. Turn left and go right at Tyranith's message where you started the hub. Follow the hall back to the room with the holy water basin.

Walk up to the basin to cleanse the Holy Cross. Collect it and return to the teleporter. Enter it and approach the defiled altar to cleanse it and reveal a secret passage.