This Bicorn is found in the Northern most room of the Royal Crypt. Climb down into the Underground using the Northern entrance in the Basilica District. This is a dark room with Underground Creatures that can block or slow Arno. Light your Lantern and head into the room. Along the Eastern wall you can get the Bicorn. Get into the wooden beams coming from the wall. Drop down onto the platform below them to claim the final Bicorn.