What is a Rat King?

My uncle recommended a fantastic work of teen fiction called Rotters by Daniel Kraus. This is a book about a teenage boy whose mother has died and he is reunited with the father he has never met. His father inducts the young boy into a ancient and secret society of grave robbers.

It was through this book I learned a rat king is not just an invention for children’s ballet but an actual thing. A horrifyingly disgusting thing.

At one point in the novel, father and son are digging up a grave when they come upon a nest of rats, which turns out to be a rat king. According to Wikipedia, a rat king is a collection of rats whose tails are bound together, through injury, excrement, or foreign substance, or all three. There is a famous rat king preserved in a museum in Hamelin, where they appear to have had more than one rat problem if one gives any credence to the legends of the Pied Piper of Hamelin.