The Immortal Sword: The Piece Maker can be found on the second floor of the Clothing Store and purchased for 122 Bucks. Purchasing this hat will allow you to wear this hat only in Adopt Me!, unless you buy this item from the Roblox Catalog.
The description of this item on the Roblox Catalog reads
"Rest in pieces. This sword hangs on your back, so it's always ready for action. It's a hat though... so it's never really ready for action."