User blog comment:Jaredthefox92/A question/@comment-5940646-20171218214051/@comment-4029377-20171219175129

That's what I had in mind, either they just worship Nurgle and the rat theme is basically played out attributed to him with Skaven like themes, they worship them both and they're a very obscure cult, or basically they believe Horned Rat is a form of Nurgle for whatever reason. However they would basically be considered Nurglites in the grand picture. They could be a mixture of Chaos Beastmen (yes, those are canon in 40k), mutants who get rat mutations, or they just cosplay as that as part of their theme like those type of cultists who have animal horns and whatnot.

Their cult would also enjoy cannibalism and are found in a lot of sewers and Hive Cities. I don't know if Astartes would be a part of this but of course they would work with Chaos Space Marines. They also have a lot of Nurgle goodies for a non-astartes faction as well, (implying they're somewhat successful at spreading plague and whatnot.)