There is a lot to do in Genshin Impact, like hunting down pirate treasure for Kaeya, one of the Knights. He will need you to find some treasure for him, because it is something that he cannot do on his own. As a member of the Knights, he cannot be considered to have any connection with the black market.
For the first part of the quest, you need to visit the Adventurers Guild and learn about the Arcadian Ruins. All you have to do here is follow the waypoints and talk with the NPC outside of the guild. They don't know anything, so you can go back to Kaeya. From there you'll need to meet his informant, so follow the waypoint out of town to the edge of the Weeping Forest.
The informant will arrive, but has a few enemies behind him. Kill them all, then speak with the information to get a clue and a picture. The clue is as follows:
In the city by the lake, three six-armed giants lived. Two to guard the Archon statue and one to guard the treasure.
Riddle of the Arcadian ruins
If you look across the water in Mondstadt, you'll notice three large windmills that each have six sails. These are your three giants. Go to the Windmill solo and you'll need to find a way to get to the top. The easiest thing to do is climb into the Knights building, marked on the map by the sword icon, and jump off the top. It may seem like an impossible climb, but you can rest on the ledge between the floors, allowing your stamina to recover. When you reach the top, jump and slide on the upper walkway around the windmill and open the chest you will find there.
If you were betting on another puzzle, you were right. The next riddle is below:
She doesn't like him at all. The passion rushing through the clear spring is just a forehead. When she is cold and alone, her true heart is revealed.
Second riddle of the Arcadian ruins
You can go visit Kaeya to get a clue, and he'll tell you to head to Springvale, although that's pretty clearly marked on a map that you also get in the chest. You'll also need a character with the Cryo ability, so make sure Kaeya is on your team and then head to the next location.
Head to the location marked on the map, then climb the cliffs until you reach a waterfall. Here you will find three small ornate pillars that you need to freeze with Kaeya's abilities. When they are all frozen, a chest will appear containing a final card.
It will reveal an area near the Falcon Coast, which will also be marked on your in-game map. Head there and you will find the Arcadian Ruins. It's dungeon time so make sure you party with Kaeya and Lisa because Cryo and Elctro are really powerful here. Just play through the dungeon, then take out the final boss and the quest will be over.