During the seventeenth Devil May Cry DMC Mission: Furnace of Souls, you can collect one secret key: Ivory Key. DMC Ivory Key can be found in a hidden room, behind a blue vine door.
You can reach the door shortly after you defeat creatures like Butcher and Witch on the first huge platform.
Secret Key 1 – Ivory KeyLocation: Mission 17 – Furnace of Souls
Required ability/weapon: None
Additional Information: Upon defeating creatures like Butcher and Witch on the first big platform, the road will take you to the another one, a pretty long platform. As soon as you jump on the second platform, turn right and you will notice a blue ring in the distance. Angel Lift to it, in order to reach the platform with a dead tree on it. From there, climb up on the upper level, destroy the blue vines, and proceed to Ivory Key.