Skullders are a particular species of large spider that use skulls as shells, much like crustaceans. A Skullder will constantly change between skulls during its lifetime, either due to size increase or decay of the skull itself.
These creatures have found many uses for their shells. It allows them to remain unseen in many of Ailenor’s gloomier locations, perfect for lying in wait for their prey. It also gives them extra resilience against their predators and the environment.