Answer:
What lies in a tunnel of darkness. That can only attack when pulled back?
Put into a pit, locked beneath a grate, guarded through the night, yet it still goes out.
What an fill a room but takes up no space?
It is a sound of empty, speak and it'll talk back plenty. But all the more you yack, you'll get the same words back.
I build up castles. I tear down mountains. I make some men blind. I help others to see.
I break away from my pack. I create holes in my victims. I can travel for miles and then disappear. I am part of a dying breed. What am I?