The Den of Thieves is a hidden achievement in Genshin Impact you can obtain once you unlock a certain area, The Chasm: Underground Mines in Liyue.
The said area is a chamber north of The Chasm: Main Mining Area that is locked out by a metal gate. This chamber is a room owned by the Treasure Hoarders where they keep their stash of valuables that they collect in The Chasm.
Inside it are treasure chests and one of the Lumenspar you need to collect. The lever that opens this chamber is on the other side of the gate.
This chamber is next to where you first found and saved Clitopho.
To access this Treasure Hoarder den, you need to find two map fragments kept in two different Treasure Hoarder camps. One camp is located northeast of Ad-Hoc Main Tunnel in The Chasm: Underground Mines, and the other is in Lumberpick Valley, north of The Chasm.
Locations of 2 Treasure Map Fragments
The Chasm: Underground Mines
To get the Treasure Map Fragment in the Chasm: Underground Mines, you can travel to the eastern Teleport Waypoint in the Ad-Hoc Main Tunnel and walk down the wooden platform to the northeastern part of the map.
When you climb up a wooden ladder, you will be attacked by a Treasure Hoarder, and right past that enemy, more bandits will be around.
There will be two camping tents in this area, and the one that has the Treasure Map Fragment is the tent in the upper section. Go near the column where a bag and books are hanging, and you will find the map fragment in it.
To get the other Treasure Map Fragment, travel to the Teleport Waypoint in Lumberpick Valley and follow that valley path until you get to the large or main Treasure Hoarder camp.
You will find the Treasure Map fragment on the pile of bags and books under the middle tent.
This camp is also part of the Undetected Infiltration World Quest, and completing it may also have you pick up the map fragment. To learn more about this world quest, please follow our guide by clicking this link.
Treasure Hoarders’ Treasure Map
Once you have both map fragments, they will automatically turn into a Treasure Hoarders’ Treasure Map. This will show a map of The Chasm with a spot marked on it, another camp of the Treasure Hoarders.
The area is a little south between Tiangong Gorge and Glaze Peak. To get there, you can teleport to the Teleport Waypoint Tiangong Gorge and step on the nearest spoutrock to boost your jumping ability to get up to the upper area.
Walk up the broken wooden platform to the higher area and look for another spoutrock to jump up to the camp.
Treasure Hoarders will be staying there, so be prepared to fight. After defeating the opponents, go to the tent and look at a haystack.
Based on the treasure map, you need to burn the haystack, which will reveal a hidden door.
Enter this door, and it will lead you to the locked chamber in the Chasm: Underground Mines.
Getting in the chamber will unlock the Den of Thieves achievement and give you access to an Exquisite and a Precious Treasure Chests. You can now open the metal gate that is locking the room out and get 1 Lumenspar.