Goddess Harp is a post-Moon Lord tier Expert Mode-exclusive accessory that is obtained from the Treasure Bag dropped by Olympian Athena and it is the stronger version of Seraph Harp. While equipped, it summons Athena herself to fight for you. She attacks by shooting razor sharp feathers at a quick rate, and she also casts charged owl runes that cast lightning bolts.
This accessory is the only accessory in this mod that summons a boss to fight for you. This might show that Athena supports you as an ally despite she wants to fight you again regardless.