Necrocrawlers are wild creatures made up out of the radiation within the Old World. They look slightly human due to them ingesting human flesh for a food source, the remains of what they intake end up creating some of the body parts which they have on their gooey sticky bodies. Due to all the rotting flesh they have on them, their smell lingers in the air when they're around. That's how one can sense that a Necrocrawler is nearby; this is essentially helpful since Necrocrawlers are only visible during night, which makes them very hard to see physically. They don't linger in the daytime due to their overall fear of light, mostly because it's their weakness.
They don't have a face, or ears for that matter. Their bodies are long and gooey, they are able to stretch their bodies until it isn't possible anymore. However the more they move around, the more their goo will track to the ground, making them slowly loose their ability to stretch. Necrocrawlers have big mouths centered in the middle on the back of their heads with very sharp teeth. This helps them trick people or other creatures into thinking they're sort of harmless if they can see them from afar in the dark. Once their prey is confused or unaware of their presence, they twist their necks around very quickly and open their big mouths in order to eat whatever they are choosing to ingest. Also, instead of having arms and legs, they have arms as legs and arms as arms; so basically they use four arms to crawl around.
Necrocrawlers came to exist when diseases broke out and the world started becoming more polluted. The goo and radiation would cover any decaying body that they were able to find on sight, and this would help them grow their figure as well as grow in general to become something more monstrous. It was helpful in a way since it cleaned up dead decaying matter much faster than bacteria breaking it down itself. They also can escape light through inter-dimensional wormholes, which leads them back to their den where they stay. They can easily grab their prey and stuff them through the worm holes too, which makes them dangerous to go near since there's really no way out once they're in the Flesh room.
The Flesh room is what it sounds like. It's a room made out of the skin and blood of decaying human flesh.
They feed on the remains within their room when the sun is out as they constantly eat to stay alive. They also need the blood in order to keep their goo in a healthy state. Now the way Necrocrawlers multiply is from the Flesh room. They form like growing bacteria in there. Necrocrawlers goo basically forms other necrocrawlers once enough of it hits the flesh within the room and starts having a craving for the decaying matter too. Due to their unnatural way of reproducing, they are not labelled any gender.