THE LOST CITY - CHAMBER OF EXORCISM CHALLENGE TOMB
CHALLENGE TOMB ENTRANCE: The entrance to the final Challenge Tomb is in the clearing at the northeast corner of the city. There are cave and bear icons in that area on the level map, so you can guess what's ahead. Go through the large archway and follow the tunnel to a cave with a huge bear. By now you're probably a pro at fighting bears. Hit it with a few grenade arrows—or a combination of poison arrows and rifle or shotgun blasts—and that should do it. Collect your 2 bear skins (he's huuuuge!), plus a few deer hides from the bear's kill. Then break down the crumbling wall at the far end of the cave. (screenshots)
TUNNELS AND CAVERNS LEADING TO THE TOMB: Follow the tunnel to a large cavern, where you'll find a stack of gold coins and a box containing a relic (1/10), a Blue Glass Vase from the Icons of Kitezh set. Exit out the other side of the cave and continue to another cavern with a ledge overlooking a pool. Dive in, swim across to the far right, and dig up a survival cache (3/6) buried behind a natural stone column. The flashing light reveals its position, but you can also wait until it's marked on your map to get it.) After digging up the cache, turn to face the ledge where you entered. You'll then spot a container of cloth, the Explorer Satchel (1/2) for this area, and another relic (2/10), a Soldier's Pendant from The Athanatoi set. Swim across the cavern and through the flooded tunnel to the southeast. When you're able to surface, climb out of the water on the right and follow the path down to the Geothermal Cavern Base Camp. Grab some fire arrows and dig up another survival cache (4/6) in the corner near the campfire. (screenshots)
Dive into the pool near the campsite and swim down to the right (west). Follow the flooded passageway to a small side chamber. Here you'll find the third survival cache (5/6) in the tomb area, a container with cloth and Byzantine coins, more fire arrows, and an Archivist Map (2/2). (screenshots)
When you have everything, swim back out to the main pool. Then climb the stairs to the right (southeast) and follow the path down into a huge cavern. At the end of the path is a gate with a vent spewing flammable gas. Stand at a safe distance and shoot a fire arrow into the gas vent to destroy the gate so you can enter. (screenshots)
THE CHAMBER OF EXORCISM: Inside the puzzle room, follow the upper ledge around to the left to find more fire arrows and a document (4/8), Good Intentions from the Chamber of Exorcism series (Greek). Now go all the way around to the other side of the room and down the stairs. Pick up a stack of gold coins on the way down. There's also a second document (5/8) sitting on the floor on the left side of the room when facing the round, metal hatch. It is A Moment of Clarity from the Chamber of Exorcism series (Greek). (screenshots)
Now for the puzzle. The mechanism in the lower room has 2 rope-wrapped levers which together drain the water from the pool and open the round hatch, causing flammable gas to jet into the room. The hatch only stays open temporarily, however. There's also a cage submerged in the pool, a winch that Lara can crank with her axe, and a rope-wrapped counterweight off to the right. (screenshots)
The first step is to free the cage from the pool. To do this, use rope arrows to pull both levers. Doing so drains the pool in the floor and opens the round hatch, inundating the room with flammable gas. Before the hatch closes and water rises again, quickly move down the ramp into the drained pool and use your climbing axe to detach the hook that anchors the cage to the floor. Return up the ramp and move around to the winch. Crank it to raise the cage out of the pool. Now go into the corner to the right of the round hatch. Turn around and use a rope arrow to pull the counterweight toward you. Doing so swings the cage around toward the middle of the room. When you have the cage in the correct position, Lara says, "That looks about right." (screenshots)
Move back to the other side of the pool and use rope arrows to pull the 2 levers again. This opens the hatch and fills the room with flammable gas. As soon as you've pulled the second lever, run up the stairs nearly to the top so you're out of the way of the main gas cloud. Then shoot a fire arrow into the gas, causing an explosion that blasts the cage through the codex room gate. Now you can enter, pick up various supplies, and examine the codex to unlock GREEK FIRE. This upgrade is equipped automatically. Your normal fire arrows and molotovs now burn blue and are hot enough to melt armor. You also gain XP, and if it's the first time beating the tomb receive 10,000 Expedition Credits. (screenshots)
RETURN TO THE BASE CAMP: When you have everything here, return up the stairs and past the gas vent in the doorway. You can either run through the gas, getting a little dizzy in the process, or stand back, explode the gas with a fire arrow, and then move through before the gas can build up again. Head up the path and then swim across the pool to the Geothermal Cavern Base Camp.
If you've raided all 9 challenge tombs in the game, you will also have completed the Ancient Secrets Mission, begun in the Geothermal Valley. So you may want to fast travel to the Ruins Encampment Base Camp in the GEOTHERMAL VALLEY to revisit the Remnant herbalist who gave you this mission. Her tower is just up the hill from the campfire. (screenshot/map) She'll give you 10 Byzantine coins when you arrive, but you'll need to press Interact to talk to her once more to complete the mission. In addition to XP, you'll receive the Sacra Umbra Outfit. You can then fast travel back to either of THE LOST CITY base camps to continue.
To get back to the city streets from the Chamber of Exorcism, either fast travel to the Threshold of Kitezh Base Camp and slide down the zip lines or make your way back through the caverns the way you came.