Hell-Rats are found in Evil Dave's basement in Edgeville and can only be accessed after starting the Evil Dave subquest in Recipe for Disaster. Hell-rats are caught by having your cat chase and catch them, just like normal rats. After a cat or kitten manages to kill a couple of hell-rats, it will become a hellcat or hell-kitten. The cat may be changed back to normal by feeding it milk or by talking to Dave's mother.
When successfully caught, the rats always drop ashes and a random spice. The spice will have 1 to 4 doses and may be red, yellow, orange, or brown. These spices may be mixed with stews to create Spicy stews.
During the quest, Evil Dave claims that he is trying to raise a demon army and had tried to start off small with imps. However, Evil Dave ended up with hell-rats, which were supposed to be the lowest form of demons.
Note: A full grown cat has a much higher chance of success at killing hell-rats versus a kitten. Kittens only have about a 5% chance of catching Hell-Rats, and as such are not advised.
Hell-rats have a bigger variant, Hell-rat Behemoth, which will give you a 4-dose spice when succesfully killed. There are 4 Hell-Rat Behemoths around the room, each protecting a different spice, use the respective curtain to obtain the desired spice. Your cat will be killing it for you. It is strongly suggested to bring any sort of fish to heal your cat during the fight.
Note that if your cat dies during the fight, you will lose him. A way to save your cat when fearing his death is to run away from the fight. The cat will then follow you again, losing the fight however.
It does not matter which type of fish you give your cat during the fight, since they all heal the same quantity of health.