Columbus, Ohio's Twenty One Pilots might just be the biggest cult band in the world. Thanks to their mythmaking and music, they've inspired in their 'Skeleton Clique' the kind of superfandom normally reserved for space epics or fighting fantasies. But behind the flights of fancy, there's humanity. Gary Ryan meets the band as they kick off their stunning UK arena tour in Birmingham, and hears how family, dementia and anxiety fuel their genre-hopping songs.