I use the item and get sent to the arena, use it again to summon the boss, beat the evershining light out of her, and then she just... leaves. Drops some hearts and that's all, no prism, no loot, nothing, and the boss checklist doesn't mark her as defeated (although Asphodene does remark on her defeat and gives her essence).
So what am I doing wrong? Is there some special requirements to this fight, like, only beat her using mod's own weapons (I use Calamity's Galaxia) or finish her with Nova or whatev?