This beautiful shield has a chance to drop from the rare elite Eliminator Sotiros within the Citadel of Loyalty in Bastion. Sotiros is summoned along with another rare elite, Larionrider Orstus, so be sure to bring friends -- including a tank and healer for dispels -- along!
Summoning the pair requires a player who's a member of the Kyrian covenant that has unlocked Rank 3 of the Anima Conductor and has anima channeling to the Citadel of Loyalty. That person then needs to ring the Black Bell. If you're not a Kyrian, you can try looking for a posting in the Custom Premade section of Group Finder since the mobs cannot be soloed at this time.
Note: The shield can drop for anyone regardless of their covenant choice.