Describe this other world and evince the mood that this world effuses. Try to use concrete descriptions (so don't just piece together a bunch of emotional adjectives in a sentence). As Chekov says, don't tell me the moon is shining, show me the glint of light on broken glass. Prose, poetry; any format is fine, but again, you shouldn't have to state what feeling it is you're evoking. Poetry isn't an excuse!