It knocked off his helmet, which rolled to the floor, making the strange signs painted inside visible.

The moment it was off, it was clear what those symbols were meant to block, because Nico's "take out the trash" spell suddenly made him disappear.

"Those symbols, they - they could see us and were immune to your spell," Dipper said as he ran out on the fire escape. "And he knew about the book - how did he know about he book?"