Matured Demon
Gross. Really gross.
That'd be the perfect description for the thing he's seeing right now.
A demon whose body is like a big, black gelatinous mass that writhes as it crawls the tunnels. It's covered in warts that constantly leak out black and yellow-ish colored goo. It's like a giant maggot with the way how its body pulses with every move.
What makes it more disgusting is the fact that, whatever this thing is, has a cannibalistic streak. Ashton could make out several demons being savored inside its mouth as it writhes forward. He could even hear the screeches of its victims which made his skin crawl.
It has a foul appearance, presence, and smell. It's disgusting to look at and every single action it does is nightmare-inducing. But, nothing could top the one thing it did which nearly caused Ashton's knees to buckle in fear.
The creature stopped at one point, and Ashton couldn't help but notice that it stopped precisely at where he disappeared.