Necro as I started using it in the late '80's and it became to be used is as the ultimate superlative. You have big, bigger, biggest, and necro. No matter what is the subject of conversation, to refer to it as necro implies the grandest qualities. It also refers to the grim as related earlier.
Man watch out the neighbor's necro-dog is loose! The melted cup from the dishwasher becomes the necro-cup. A diamond studded Rolex would be a necro-watch. Did you see that dude with the necro-watch on? A friend of mine had a viscious, old female cat that would kill birds eating out of her outside food dish. We would see the birds eating out of her dish, later we would see feathers all over the dish. We called her necro-cat.