Main Mission 14 will pit you against three bosses you fought earlier in the game and after defeating all three, you will exit via a door to get back to the real world. Follow the trail all the way to the end, where some roots will be blocking the exit. You will need to head to the end of the side path to destroy a blood clot to clear the roots, but the red glow for the Secret Mission will be behind that destroyed blood clot.
There are two methods to finish this Secret Mission, the first of which involves using Nightmare and the Promotion skill. The objective is to reach the Blue Orb Fragment without touching the ground and riding Nightmare will fulfill the challenge before you. Start off by jumping off the ledge and summoning Nightmare, but make sure you hitch a ride before you happen to hit the ground. Once you’re riding Nightmare, you simply need to move forward until you manage to get to the end. Note that you will need a good amount of Devil Trigger gauge to pull this off, so it depends on how many Purple Orbs/Fragments you have collected along the way.
The other method will take into account the slight movement you get while swinging your cane in the air. Jump (just a single one) off the ledge and immediately start swinging your cane with the Circle/B button. Keep doing this until you start losing height and it looks like you clear the ceiling ahead of you. Once you are clear of the ceiling, perform a Gambit (R1/RB + Circle/B) to teleport forward. After doing that, you can double jump and glide with Griffon to hopefully land right on the Blue Orb Fragment.