Phantoms

Phantoms are creatures that possess two forms, a physical and ethereal form. Ethereal forms are unable to interact with the entire world excluding magic spells. Likewise, only magic can affect ethereal forms. A physical form is basically an average being, excluding the fact that the body is merely a shell with an unattached mind controlling it. Switching from ethereal form to physical form requires a large amount of magical power, but switching from physical to ethereal is practically effortless.