Secret 9 is a secret that's a black cube.
Location[]
From the entrance to the Debug World, go right, right, right, up, and you'll see rotating black squares. Jump in the middle of the squares 20 times, and the gates will open.
Alternatively, you can get past the gate with the Wiggle Glitch by wiggling along the left edge of the screen with the rotating black squares.
Description[]
"It's black."