Adso, the light.

Venantius, following the instructions on the parchment, entered the forbidden library and found the book. He took it back to his desk and began to read it. After scribbling down those mysterious quotations, he died with a black stain on his finger. The assistant discovered the body, and dragged it down to the pigpen to avert suspicion falling on him. But he left his "autograph" behind.