This Secret Mission is located at Mission 14: Diverging Point: V, after the area where the player fights the Mirage Goliath, Mirage Artemis and Miraggio Angelo, there will be two Blood Clots that must be destroyed in order to progress. Once the second one is destroyed, walk forward, past it, rather than go back and finish the mission. The glowing point will be in that area, then simply turn the camera around and find the pattern.
The easiest way to complete this Secret Mission is by purchasing the "Promotion" move which allows V to ride Nightmare. The player spawns with the perfect amount of Devil Trigger to call the creature, then simply get on Nightmare's back and walk towards the Blue Orb fragment.
If the player does not have Promotion, the mission can be completed fairly easily by circling around on the starting platform until Quickplay activates, using it to long-jump by steering off the edge, then gliding with Griffon when V has fallen far enough not to get stuck on the archway and finishing up with Gambit to teleport forwards to the Orb.