In Where Winds Meet, Echoes of Old Battle is a side quest that involves a few puzzles, and rewards you with a new skill. Here's everything you need to get started with it:

Rewards

Item Quantity
Touch of Death x1
Wind Beneath Wings Page x4
Medicinal Tales x1
Lv. 2 Ebon Iron x6
Oscillating Jade x2
Echo Jade x50
Qinghe Exploration x88 points
Enlightenment Points x100
Character EXP x13,000
Zhou Coins x13,000

Location

The quest begins by heading to the Northern Vow Ruins located at Battlecrest Slope in the Qinghe region. 

Walkthrough

Step 1

Upon arrival at the marked location, interact with the eavesdropper near the gate.

Step 2

Push the gate open and eavesdrop on the guards.

Step 3

Then blow up the gunpowder barrels.

Step 4

Take down the enemies in the area, including the more formidable Zhang Bao.

Step 5

Open the chest in the area.

This will reveal the location of the contraptions that need to be lit.

Two of them should already be lit.

Step 6

Light the contraption on top that is nearer to the chest you previously opened.

This will cause a mechanism to open at the center of the training ground.

Step 7

Use the 'Mighty Drop' skill on this to expose stairs descending underground.

Step 8

Follow the stairs until you reach a corpse. And get the clue (will be useful for the next wall puzzle)

Step 9

From the corpse, move left to descend further.

You will encounter a large statue where performing the Yanyun Oath gesture activates a wall puzzle.

Step 10

This puzzle features two walls with blocks inscribed with Chinese characters. Pulling out blocks causes them to illuminate, while pushing them back makes them dim.

The solution requires lighting up the blocks matching the characters from the Yanyun Oath clue you obtained earlier.

The characters on the left are for the left wall (as you enter). Characters on the right are for the right wall.

Step 11

Once the puzzle is solved, the door will budge. Go through it.

Step 12

Avoid the rope trap in your way. Then use the wooden path to reach the treasure chest.

Step 13

On the right, drop to the area with the water wheel and waterfall. Go through the waterfall and activate the mechanism to spin the water wheel.

This will unlock the final stone gate.

Step 14

Inside the chamber, open the treasure left by Northern Vow soldiers and read the inscriptions on the walls to conclude the quest.

And that's it! You would have completed the Echoes of Old Battle side quest and earned your rewards.