For some of the really dark areas, I had to put on up to 8 layers of the same marker color and also used some black up in the ceiling and the inside of the furnace..
For some parts, I also used pencil crayons to put in hi-lites or dark shadows and a bit of texture.
This layout is based on a really scary dream that I had when I was a kid, about 9 years old. The water heater in the basement lured me down by placing a model kit of a Bobby Orr hockey stick (lame I know), (and it really dates me). I went down to get it and heard a noise coming form the furnace room. As I went over to check it out, the water heater came to life, grabbed me by the ankle and threw me against the wall. I tried to get away but he caught me again... thankfully, I woke up. (I can't remember if I peed my bed or not.)