The Mysterious Part B is a Key Item in Xenoblade Chronicles 2. It can be found in a buried Treasure Trove in the middle of the Brionac Occupied Zone in the Empire of Mor Ardain. It will not appear until you have deciphered the Mor Ardain Murals during The Riddle on the Wall. Level 5 "Earth Mastery" and level 2 "Keen Eye" Field Skills are required to excavate the chest, and level 6 "Lockpicking" to open it.
Gallery[]
Approximate location of Mysterious Part B's treasure trove