You can find this minigame in Sascilia Steppes. You can also talk to Kat Rojo in Atlantis to get ported to the minigame.
In the begin of the minigame you will get ported in one of four entrance halls. Each will let you start in a different spot of the minigame. Sneak through the dungeon-like game without alarming the enemies and light all 3 Elemental Lamps. Then go to the middle to collect all Treasure Chests.
Your character skills will be disabled. Instead you will have the following skills:
|Invisibility||Be invisible for 15 seconds. Ghost Guard dog can still see you! Reduces movement speed under usage.||30 sec|
|Feign Death||Fall to the floor, leaving combat. Lasts up to 5 minutes. You can get up earlier by moving.||1 min|
|Snipe||Kill an enemy one hit.||3 min|
|Binding Chain||Root your enemy for 10 seconds. Ghost archers can still attack!||20 sec|
Find these skills in power-up boxes. They can be used once.
|Azure Rejuvenation||Restore 100% HP||15 sec|
|Ghost Cavalier||Transform into a Ghost Cavalier for 15 seconds, leaving combat and all enemies ignore you||3 sec|
|Protective Shield||Become invincible for 15 seconds.||none|
|Poison Gas Trap||Lay down a trap for enemies that deals 20% of their max health over 10 seconds.||3 seconds|
|Treasure Seeker||Finish the minigame once|
|Ghost Cavalier Criminal||Finish the minigame 10 times|
|Rune Investigation Team Hero||Trade 30 Ruins Investigation Medals|
|Treasure Hunter||Finish the minigame 50 times|
|Owner of the Treasure Mountain||Finish the minigame 100 times|