Monsters roaming the polluted earth outside the cities are called Filth Monsters (or also otherwise known as Pollutant Beast or Contaminoids). In the novels, Layfon and Kalian encountered an enormous and extremely powerful one that speaks the human language. According to it, pollutant beasts are also a creation of human. Their main food sources are the pollutants they breathe into the atmosphere, humans and themselves if the former cannot be obtained. They come in many forms and sizes:
- Larva: The first state of the Filth Monsters. As ravenous feeders, they will eat any living thing they can find. They are overwhelming in number although they are easily dispatched by skilled Kei users and expert Military Artists. They proved to be difficult for the students of Zuellni when they first encountered in the novel.
- Queen: This is the second form of Filth monsters. A Filth monster that lays eggs that hatch into Larva.
- Mature: This is the third form of filth monsters. A Filth Monster which has moved past its ability to procreate, the longer they live the more powerful they become. In their mature state, Filth Monsters are able to fly. In Glendan, there exists a tradition that if a Mature Level Filth Monster is engaged by the Heaven's Blade Receivers and manages to escape alive, then it is given a name. In the anime, one such example is Limbeekoon, though it was defeated by Lintence, Savaris and Layfon, it has a counterpart in the Light Novel called Behemoth.