Unlike all other realm-creatures, the description of Infernal Creatures says
"All infernal creatures can put off their physical forms and revert to
being pure spirits at will."
(emphasis mine)
Assuming that RoP:I will not throw a wrench in the works and water down
that "all" (though I hope it does!), in the generic ArM worldview, are
such spirit-forms invisible or not?
Cheers,
--d