SOVIET INSTALLATION - VOICE OF GOD CHALLENGE TOMB
The entrance to this tomb is inside the SNOW LEOPARD CAVE, which is covered in the main walkthrough. Once you've defeated the cat, you can safely enter its lair. Use a rope arrow to remove the rope-wrapped barrier at the back of the cave. Then pry open the metal door, go through, and follow the tunnel to the Frozen Gorge Base Camp.
CANYON LEADING TO THE CHALLENGE TOMB: After making your way to the Frozen Gorge Base Camp, as shown in the previous section, exit through the opening beyond the campfire and turn left. Squeeze through a narrow gap in the rocks. Jump across the wide gap in the ledge. Then scramble up the wall on the left to grab a narrow ledge. Traverse around to the right until you can climb upward, then right some more. Jump up and latch onto the climbable wall with your axes. Then climb upward, jump across the gap to the next climbable area on the right. Then continue to the ledge above. Cross a narrow stone bridge above the canyon. Take a running jump to latch onto the next climbing wall. Climb down and around to the right for your first glimpse of the tomb. Then continue to the right until you can drop down onto a flat, snowy ledge. Move to the far end of this ledge and jump to grab a handhold on the cliff face ahead. (screenshots)
Climb/jump to the right along the narrow handholds until you can't go any farther... ..and Lara is hanging from a ledge with a snow-covered bush growing on it. Take a long leap to the right and try to latch onto the next climbable wall. The rock crumbles, and Lara loses her grip. press Interact as she falls to latch onto the wall below. Climb to the ledge above on the right and follow it toward the tomb entrance. You'll pass a ledge on the left. Ignore it for now. You'll return this way after exploring the tomb. Continue forward onto another narrow stone bridge. The bridge collapses as you try to cross so you must jump forward and press Interact again to latch onto the climbable wall ahead. From there, jump over to the next section of rough wall, climb to the top, and traverse along the narrow handhold to the right. Then continue climbing to the right until you can pull up onto the ledge in front of the tomb. (screenshots)
VOICE OF GOD CHALLENGE TOMB: When you first climb onto the ledge in front of the challenge tomb, there's an Explorer Satchel (3/6) sitting directly in front of you. Examine it to reveal the location of several survival caches on your map. The first survival cache (11/21) is just behind you. Dig it up. Before entering the tomb, head down the hill to find a relic (10/18) sitting near the start of the broken wooden walkway. It is Maniples, a ceremonial cloth from the Tokens of Faith set. (screenshots)
NOTE: If you have the Baba Yaga: Temple of the Witch DLC, the path to the Wicked Vale continues beyond this relic. In order to complete this adventure, you will first need to locate and speak with the NPC Nadia. She can be found in the SAWMILL area of the SOVIET INSTALLATION after completing the GULAG. In order to get all of the collectibles in the Wicked Vale, you'll need the grapple axe (wire spool) from the ACROPOLIS level. A shotgun will also be useful but is not required.
Now enter the tomb. Pick up 2 gold coins sitting on the floor. Then dig up the next survival cache (12/21), which is buried just to the left of the large inner door. On the left side of the room, there's an Archivist Map (6/9) that reveals the location of the documents in this area. One of those documents (17/26), The Scout from a series called Voice of God, is sitting on the next table. Read it for XP and Greek proficiency. (screenshots)
Now use your axe to turn the crank near the outer door in order to close it. This also raises the counterweight on the other side of the room. Scramble up the closed door and climb over to the archway on the left. In the next room, drop down onto the ledge on the right. There's another document (18/26), Mountain Song from the Voice of God series, sitting on a shelf. This one also gives you XP and Greek. There's a second crank here as well, but using it does no good yet. for now, hop across the gap and use your axe to smash through the bricked-up archway to get back into the main room. (screenshots)
Use the first crank again to re-open the outer door and simultaneously lower the counterweight in the other room. Return through the broken archway to the side room. Stand near the second crank and shoot a rope arrow into the counterweight to anchor it to the rope spool. Now use your axe to turn the crank, pulling the counterweight toward you as far as it will go. The crank should then lock into place. Move around to the right side of the crank and watch and listen as the wind blows across the room. You'll notice there are pauses between each strong gust. Just as the wind is starting to pick up, press Interact to release the counterweight. If your timing is right, the wind will blow the counterweight across the room, smashing the ice and freeing a second counterweight there. If it doesn't work the first time, try again until it does. (screenshots)
Return to the main room and use the third crank to open the inner door. Head down the steps to find a codex that grants you a new skill: Inner Strength. This automatically restores Lara's health once per battle if she is critically injured. You'll also receive XP and 10,000 Expedition Credits if this is the first time you completed this tomb. (screenshots)
RETURN TO THE SOVIET INSTALLATION: Go back up the steps, through the tomb, and out the other side. To get back to the Soviet Installation, start by the anchoring a rope line from the rope-wrapped post to the right of the doorway to the beam on the other side of the canyon. Use the line to cross. Then, instead of following the path back the way you came, scramble onto the ledge above on the right. There's a container of cloth here and a survival cache (13/21) a little farther on, buried near the right edge of the ledge with the rope-wrapped post. Dig up the cache and then use a rope arrow to create another line so you can slide down to another ledge, where you'll find a relic (11/18), a Censer from the Time of Adaptation set. (screenshots)
Climb the handholds and traverse around to the right to reach the ledge above the desiccated corpse. Stand near the post and fire a rope arrow across the canyon to anchor another zip line. Slide down, move forward, and drop down onto the ledge below. Turn right, jump across the gap, and squeeze through the narrow opening in the rocks to find yourself back at the Frozen Gorge Base Camp. You can then return through the tunnel to the LEOPARD CAVE and continue with the main walkthrough, or fast travel to any other base camp. (screenshots)
†UPDATE HISTORY: 11/29/15 - First draft of walkthrough posted online.
4/24/16 - Made a few minor tweaks, including switching the order in which the two documents are collected.
10/31/18 - Added notes about useful gear for the Baba Yaga DLC mission, thanks to a nudge from Kernighan.