The Riddle on the Wall

The Riddle on the Wall is a in Xenoblade Chronicles 2. It can be received from Erebos at Tretl Windmill Plaza in Torigoth, Gormott Province, after examining a wall painting with a Keen Eye and Ancient Wisdom level 2 check, and provided Mysterious Note was completed. Completing the quest rewards the party with two sets of Mysterious Pieces, required to complete Uncover the Truth.

Quest objectives

 * Go to the location written on the mural and get your hands on the treasure.
 * Unearthing the treasure chest requires clearing an Earth Mastery level 5 and Fortitude level 3 Field Skill check. Opening the chest requires clearing a Lockpicking level 5 Field Skill, revealing Mysterious Part A.
 * You obtained a mysterious part. Report back to Erebos.
 * Find the remaining four parts, which are hidden all over Alrest, then report back to Erebos.
 * The Mor Ardain wall painting can be found in Alba Cavanich, behind one of Nharil Central Plaza's walls. Jumping on the Admar Storage Zone's roofs is required to access it. The mural can then be examined with a Keen Eye and Ancient Wisdom level 2 Field Skill check.
 * The treasure chest can be unearthed in Brionac Occupied Zone, requiring an Earth Mastery level 5 and Keen Eye level 2 Field Skill check. Opening the chest requires clearing a Lockpicking level 6 Field Skill check, revealing Mysterious Part B.
 * The Tantal wall painting can be found in Theosoir, accessed by clearing a Wind Mastery and Leaping level 3 Field Skill check, followed by an Earth Mastery level 6 and Superstrength level 3 check, and finally an Earth Mastery level 7 and Leaping level 4 check. Afterwards, the mural can be examined with a Keen Eye and Ancient Wisdom level 3 check.
 * The treasure chest can be unearthed in Genbu Drifts, requiring an Ice Mastery level 2 and Fortitude level 4 Field Skill check. Opening the chest requires clearing a Lockpicking level 5 Field Skill check, revealing Mysterious Part C.
 * The Indol wall painting can be found in the Refugee Campgrounds, Goetuis Port, and can be examined with a Keen Eye and Ancient Wisdom level 3 check. After starting, another wall painting can be examined near Corinne's House and Messet Point, Fonsett Island, with the same requirements.
 * The treasure chest can be unearthed, using a Keen Eye and Fortitude level 3 Field Skill check, at the Isle of Sleeping Remains, which first requires going through a Malign Miasma barrier with a Dark Mastery level 4 and Ancient Wisdom level 2 check to be reached. Opening the chest requires clearing a Lockpicking level 6 Field Skill check, revealing Mysterious Part D.
 * Relic-Holder Tyrannor must be defeated at 7th Division to obtain Mysterious Part F.
 * Defeat Erebos at Tretl Windmill Plaza in Gormott.

Results
"The mural led you to six parts. But where is the ancient treasure they're supposed to be used with?"