Kuwaq Yaku – The Forge DLC
Forge of Destiny Challenge Tomb (part 6)

When you're ready to move on, enter the room in the middle of the tower.

Pick up some supplies on the left.

Then note the 3 counterweights on the right. Each of these raises one of the ledges on the outside of the tower. You'll only need to use two of them, though. Start by jumping onto the right counterweight.

BUG NOTE: I encountered a glitch here, in which Lara would not jump to grab the counterweights. Reloading the checkpoint fixed it.

As Lara sinks, the ledge outside rises.

Let go and quickly run and jump onto the middle counterweight.

The ledge between the tower and the doorway begins to rise. Don't wait. As soon as the counterweight reaches its lowest position, drop down...

...run out the door on the right, and turn left.

Run around the tower as the first ledge begins to collapse...

...and quickly jump down into the doorway before the second ledge drops out from beneath you.

NOTE: If you prefer, you can raise the left and middle ledges and run around to the left to reach the doorway, whichever works better for you.

Once you reach the doorway, follow the path up to the left.

Use the jaguar-head switch...

...to activate another gas jet.

Pick up more arrows and supplies then move out onto the wooden ledge to the south.

Now you're headed for the sunny ledge off to the south. There's a dangling pendulum to help you get there.

Shoot a fire arrow into the gas jet below the pendulum to blast it toward you.

Turn left and jump onto the pendulum as it approaches.

Ride it back to the right and jump down onto the sunny ledge, where you'll find a document (3/3), Mariana's Journal (Mam/The Hidden Past), sitting out in the open.