Short answer: Looks like Hell itself.
But how do you imagine the Infernal Demon's Starting and Region zones looking more fantasy and magical?
Purple crystals everywhere? Lava and magma formations? Purple corruption on the ground from the crystals? Many iron spikes?
Should they start as freshly made Infernal Demon? Maybe in a trial, where you have to hunt down a human and take his soul?
Let's discuss!
-EN ESPAÑOL-
Se podria empezar en una especie de lugar con lava y demás, algo estilo Izalith perdida de Dark Souls, mas que nada por que creo que quedaría chulo, pero suena bien todas las ideas que habéis mostrado en los diferentes posts :D, ya que le da variedad a los diferentes lugares donde se podria comenzar, también el lugar podria ser una especie de lugar con culto "satánico".
-IN ENGLISH-
You could start in a kind of place with lava and so on, something Lost Izalith style of Dark Souls, mostly because I think it would look cool, but it sounds good all the ideas you have shown in the different posts :D, since it gives variety to the different places where it could begin, also the place could be a kind of place with "satanic" worship.