This will happen when you've opened the body bag that contains Scarecrow. You'll be transported to an area where you face a larger version of him. Stay out of the red glare by taking cover behind walls. When you've reached a large wall and can't go any further use your explosive gel and blow the wall away. Once you've gotten past that, you'll make a few more jumps and reach a spotlight. Once you've used it Scarecrow will go away, and you'll be back where you were. Achievement will pop afterwards.
This is also story based and cannot be missed.