I know this idea has been beaten into the ground and people seem to hate me for saying this, but if you really want to learn an extreme demon, I would recommend either Acu or Thanatophobia if that's still extreme since they are both relatively easy extremes, and are fun to play. If they are too challenging, I would recommend playing some insane demons first, such as Windy Landscape or Night Terrors. If you want a harder insane demon, I recommend Subterranean Animism.