If you choose the left fork at the intersection, you'll reach a statue with glowing green eyes. Present the blue, red, and yellow spherical keys to the statue and it opens to reveal the treasure vault.
Read the tome on the nearby altar to receive your final puzzle riddle: To gain entrance to the father's tomb, feed the guardians the flames they long to consume. One seeks an ember rich in earthly splendor. The other seeks a blaze that emanates from a celestial gaze. To enter the tomb's second section and pass the open floor grates, you must solve this riddle.
First, you need to figure out what "earthly splendor" means. Earth is the clue, and if you search on the ground around the large chamber, you'll find two pressure plates far apart. If Caddoc and E'lara stand on them simultaneously, a crimson flame appears in the brazier below a statue head high up on the wall. Line up a shot so that the arrow flies through the flames and ignites the statue's mouth behind it. The first-floor grate in the other room slides shut.
Next, look for the brazier in front of the second statue head. Caddoc should push this column back into place. E'lara can then aim up at the rope holding a large shield in place in the gap between the walls. Sever the rope and the shield falls to the side, revealing moonlight. Return to the front of the statue and watch as the moonlight ignites the brazier below the statue. As with the first statue, line up a shot so that the arrow flies through the flames and sets the statue's mouth on fire. The second-floor grate grinds into place, and you can enter the main area.
Cross the floor grates and enter the door straight ahead. (The door to your right leads to the treasure chamber, but you can't access it until you defeat the Tomb Skeleton.) Even Caddoc and E'lara joke that it's time for another skeleton when they see the chamber's crypt.
Sure enough, the father of all Tomb Skeletons, the sire of the Three Brothers, rises from the crypt to challenge you. Avoid direct melee confrontation with the Tomb Skeleton; it hits very hard and can kill you in a stroke or two. Use magic at a distance if you can, and remember to block with your shield when it swings. To complicate matters, normal Skeletons pour into the chamber to support the Tomb Skeleton.
Dodge the Tomb Skeleton while you smash down the normal Skeletons and prevent them from overrunning you. Turn Dragon's Breath loose on any of the Skeletons to deal extra damage, and let loose with all your mana to stop your foes.
When the Tomb Skeleton dies, it drops its pelvic bone. Take the pelvic bone to the treasure vault door, and the door unlocks. Inside the treasure vault, scoop up all the gold and three Crystals. As a super reward for all the puzzles you completed to get here, you earn a Crystal Sword for Caddoc and a Crystal Bow for E'lara, the best weapons in the game.