Below are the quests in the Secret Pirate Treasure questline in Genshin Impact.
Kaeya’s Troubles
Obtained: Complete the The Outlander Who Caught the Wind Archon questline.
Objective: Talk to Kaeya
Head for the Knigts HQ in Mondstadt city and speak with Kaeya. When you’re done, exit the HQ and turn right then jump over the wall towards the windmill. Approach Kaeya to trigger a cutscene. When it concludes, make your way to the Adventurers’ Guild building and talk with Cyrus there choosing the “We’re looking for…” option. When you are done talking with him, return to the Knights HQ and speak with Kaeya near its entrance.
Rewards: 250 Adventure EXP, 3225 Mora, 8 Wanderer’s Advice, 3 Fine Enhancement Ore
Mystery of the Arcadian Ruins
Obtained: Complete Kaeya’s Troubles story quest.
Objective: Meet with the informant at the designated location
Exit Mondstadt and follow the road north to find the informant along the road between the city and Whispering Woods. As you approach the marker, the informant will run out of the woods being chased by enemies. Defeat them then speak with Vile.
Treasure Clue 1
The first clue reads “Three six-armed giants reside in the city in the lake. Two guard the statue, one, the treasure”. This riddle is referring to the three giant windmills in the city. Head for the Adventurer’s Guild HQ in the southern part of Mondstandt (it has a blue roof). Look for the windmill across from the guild on the other side of the stairs. Climb to the top of it to find a treasure chest containing the second clue.
Treasure Clue 2
The second clues read “She doesn’t love him at all. The passion rushing through the clear spring is just a front. When she’s cold and alone, her true heart is revealed”. This clue will lead you to a cliff overlooking the lake with a Teleport Waypoint in its center southeast of Springvale. You’ll want to look for three pillars with the ice symbol on them along the southern part of the lake. Once located, use Kaeya’s ice powers on all three of them to reveal a chest. Loot it to complete the quest.
Rewards: 250 Adventure EXP, 3225 Mora, 8 Wanderer’s Advice, 3 Fine Enhancement Ore
Kaeya’s Gain
Obtained: Complete Mystery of the Arcadian Ruins story quest
Objective: Use the map to find the buried treasure
Make your way to the eastern part of the beach on the Falcon Coast to find Lost Arcadia. Enter the domain and work your way to the end to trigger a cutscene. Defeat the boss and its adds that spawn to complete the quest.
Rewards: 300 Adventure EXP, 60 Priogems, 3850 Mora, 5 Teachings of Ballard, Harbinger of Dawn, 10 Wanderer’s Advice, 4 Fine Enhancement Ore