A shell is effectively a vessel or a body in which a powerful being, such as the Creator or the Twin Deities, can choose to inhabit in order to better fit in the rest of the world. Think of it as a civilian identity.

When inhabiting a shell, the gods must give up a significant portion of their powers as well as sacrificing a part of themselves as to remain inconspicuous to others. The amount of power required to use a shell depends on how powerful the god in question is.

Not every god necessarily needs a shell but it makes living a normal life much easier.