After breaking the 2 black octohedrons smash the right-hand bookcase to reveal a hole in the wall. Proceed, killing on the way, to a large black crystal with a small one next to it. Smash both. Smashing the large one will summon a Giant Skeleton. After, the stage portal and a door opens. Get the 2 Treasure Chests, one wood and one metal, to the south before going to the stage portal.
Doors here open and close on a schedule. Each room you step into, both doors will close and mobs will spawn. In the last small room is a secret door on the right-hand wall leading to a small room with spiders and a Treasure Chest.
Portal room has 2 large and 4 small black crystals. Same as before, the large crystals summon a Giant Skeleton when smashed.
Ahead is a room with 4 green crystals, and beyond it a room with 1 large and 4 small black crystals.
North: Enter and smash the large to destroy all 4 small. Kill the spawns and return to the green crystal room (which now has 3 and an open door). Each room you clear destroys a green crystal!
East: The room has the same 5 black crystals, with 5 spider egg sacs. Destroy the eggs without hitting the crystals. Back up and let the newly-hatched spiders come to you. Now destroy the crystals and the single Giant Skeleton.
West: 1 large and 6 small black crystals. Destroy them all!
Returning to the central room, the portal has appeared above the platform in the middle.
During the fight, if you allow him the time, Trevorius will move from one large black crystal to another, breaking them. This calls a group of mobs so it would be best to keep him too busy to break them, or pick off the Brethren and handle the adds until he runs out of large black crystals. Once Trevorius is slain any remaining large black crystals cannot be broken.