"Deep Piece" does not appear to be a standard term, item, or location within Rune Factory 4 or a known group in the 3DS ROM scene. It may be a reference to "Deep Sea" fish (like the Round Stone found in the forest log room) or "Shade Stone" pieces used for advanced forging. Rune Factory 4 [Decrypted] 3DS (UNDUB) ROM (Region Free)
A decrypted 3DS ROM is a type of file that contains the game data for Rune Factory 4, which has been decrypted and can be played on a computer or mobile device using a 3DS emulator. ROMs (Read-Only Memory) are files that contain the game data for a specific game, which can be played on a device that is not the original console. rune factory 4 decrypted 3ds undub rom reg link
If you're looking to obtain a decrypted 3DS undub ROM REG link for Rune Factory 4, here are some tips: "Deep Piece" does not appear to be a