This page in our guide to Devil May Cry 5 is dedicated to Secret Mission 06.
Where to find it: Mission 09 - Genesis
Secret Mission 06
You come across Qlipoth roots blocking a passage. Approach the roots and summon the Nightmare to destroy the blood clot. Move forward - you will get a Blue Orb Fragment and reveal Secret Mission 06.
The objective is to kill all demons within 20 seconds. The best way to do that is to summon the Nightmare and use your most powerful combos. The task isn't difficult - complete it and you will get another Blue Orb Fragment.