Black Prince

Black Prince is a shield. The user's magic increases by +300 for 5 seconds after they take dark damage.

Source
Black Prince can be purchased from Culless Munitions at Meldacio Hunter HQ (anytime) and Hammerhead (starting in Chapter 14) for 10,000 gil. It can also be found as a collectable in the Pitioss Ruins dungeon. It can be sold back to shops for 5,000 gil.