Answer:
Vikings use this for warmth.
What can be underwater, under fire, on fire and taking water all at the same time?
What lies in a tunnel of darkness. That can only attack when pulled back?
What is neither inside the house nor outside the house but a necessity for any home?
I make you weak at the worst of all times. I keep you safe, I keep you fine. I make your hands sweat. And your heart grow cold. I visit the weak, but seldom the bold.
They are many and one, they wave and they drum, Used to cover a state, they go with you everywhere.