In a tunnel of darkness lies a beast of iron. It can only attack when pulled back. What is it?

“Gun”

While a gun is related to the riddle, it is not the answer. The riddle specifically describes an object that attacks when pulled back, which refers to the action of a bullet being fired from a gun rather than the gun itself.

“Crossbow”

A crossbow does involve a 'beast' that can only attack when pulled back, but it is not made of iron in the same way a bullet is, and the riddle hints at a more direct mechanism of attack which aligns closely with a bullet.

“Arrow”

An arrow is similar to a bullet in the sense that it is shot from a weapon. However, the riddle emphasizes the iron aspect and the mechanism of being pulled back, which is more applicable to a bullet in a firearm.