It gets a little warm in the summer

This is where Siren has been for the past, to her, several decades.  In Fiore only months may have passed by at most,  but to Siren she's been burning for many years.

 It is a layer of eternal flames.   Rivers of lava snake across the land between lakes and oceans of lava.   The air here is hot enough to melt iron yet somehow still gets even hotter near the many puddles, pools or rivers of lava.   This heat overpowers resistances to heat and melts the flesh off the most resistant of creatures.   The only ones who endure it are the natives to hell that are created here.
The air itself is a constant torture to all the many Tortured Souls who dwell here.  Many demons find it a source of agony as well.  Some, however, are accustom to the heat and can live quite comfortably as far as hells go.  These are the "Natives" to the layer,  the demons that get reborn within.   Even they,  even Siren,  feel pain here, but for them they resist it and it is a part of them.   A background sensation similar to the sensation of wearing clothes.

The sky is false,  and composed of blackened stone high above in what is actually a colossal cavern.  Roiling smoke and gasses coupled with reflected light from all the fires and molten rock make it appear as though the place is in eternal sunset with a burning red color to the clouds and on the horizons.    Toxic vapors add to the background pain one feels when breathing in, but such is negligible when your face melts from the heat of in the air itself.   

If you can endure the heat and hostile environment,  you'd see blackened bodies with bubbling flesh thrashing, flailing and wailing all over the place outside of the city habitats or dens for demons that live off in the wastes.  These are Tortured Souls.  They are in a state of endless pain and suffering,  feeling every sensation to full effect.  Burning in hell.   Hot winds roll off nearby sources of lava and sear their flesh even further,   giving them a constant peak of pain that keeps coming and going,  preventing them from truly coming to terms with the pain.

Not just anyone becomes a Tortured Soul.   Each one is someone who had committed sins in life and became doomed to this afterlife,  unclaimed by any other afterlife.   Neutral, celestial, or otherwise.   Generally those who believe in this hell end up here if they earned a spot,  and those who do not have a designated after life (Due to being claimed by some deity) will also end up here if they committed the necessary sins.  

There are different layers of hell as mentioned,  and the dead get sorted out between them based on type of hell, type of sins,  and a bit of chance.  This layer is a Burning Hell,  and a type of hell that often receives the vain and unchaste.    Though such sinners also get distributed to others, such as the Flesh Pits.  A type of layer composed purely of flesh and bone.   And this layer isn't the only Burning Hell either.

This means different plots and characters can interact freely with their ideas of what hell may be,  and even go to a different Burning Hell.  All without me intruding on their concepts and them from mine.