This is 'The Witch and the Hundred Knight 2' DLC 'Additional BGM [Dear Metallia]'.
By purchasing it, you can change the in-game BGM to 'Dear Metallia'.

Software subject to license (us.playstation.com/softwarelicense). Online features require an account and are subject to terms of service and applicable privacy policy (playstationnetwork.com/terms-of-service & playstationnetwork.com/privacy-policy). One-time license fee for play on account’s designated primary PS4™ system and other PS4™ systems when signed in with that account.

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The Witch and the Hundred Knight is a trademark of Nippon Ichi Software, Inc. Licensed to and published by NIS America, Inc.