That is right.
Since you obtained the Rock Part, the Ice Part, and the Steel Part, they come together to form a music box.
You should stock up on huge apples and gummis before heading to the buried relic.
For best results, have a leader at level 100 and holding a Friend Bow.
Sometimes Mew transforms into another Pokemon.
And you should set tactics for following Pokemon to "Let's go together".
And make sure you have an empty space in order to recruit Mew.