The Dark Tower Wiki

The Tunnel Demon is a giant worm monster with many eyes, and a mouth full of large, sharp, needle-like teeth. The mouth also was lined with tentacles. It's extremely sensitive to light. It is not known how many of these creatures exist.

On their journey through the underground tunnels in the tunnel between Fedic and the Badlands, Roland Deschain and Susannah Dean found themselves being chased by one. Roland speculated that it escaped from the todash darkness through one of the doors.

Todash Monster Tunnel

Taking advantage of the monster's extreme sensitivity to light, Roland and Susannah were able to escape by leaving behind torches Susannah made from the sterno fuel they packed in their gunna, and the old bones they encountered traveling through the tunnels. The light the torches emitted were bright enough to hold the monster back, giving Roland and Susannah enough time to reach the exit.