"Welcome to Hell, mortal. You won’t enjoy it here. I will, though."
As described in the previous post, souls exist and roughly half of them, give or take, wind up in the Abyss for some time after their mortal body reached the end of its life.
The Abyss, The Underworld, Hell, whatever you want to call it, is the natural, spiritual home of the Demons. It’s important to note that it ISN’T a “hellish” place for them. It is their place, their home, quite literally constructed for and by them, so the rulers especially live quite comfortably in the pits of the Abyss.
There are thirteen individual ”pits,” each with its own ruler, and thus its own rules. The thirteen rulers of the pits are the “Demon Lords,” of which Gilrandree is one of them. He hasn’t always been the ruler of this particular pit, nor will he always be: while the Demons themselves are immortal, their power isn't. He knows one day some other demon will defeat him, take over his pit, and he’ll be relegated to the role of just another lesser demon. He also knows eternity is a long time: he’ll probably also get another turn at ruling eventually.
Each of the pits is quite different from the others: Gilrandree’s is rather traditional, lava, fire, and brimstone, souls endlessly tortured (at least until he’s done with them) in great pits of molten rock, etc… While Gil is in charge of the pit, he’s certainly not alone there: countless lesser demons serve under him, performing much of the mundane day to day tasks of tormenting the souls of the damned.
Some of those “lesser demons” are actually quite powerful, and prime candidates to try and take him down, robbing him of his pit. As such, there’s a narrow political balance the Demon Lords walk: if they try to keep their “staff” too tightly restricted, they risk rebellion, but if they give them too much freedom and power, the potential for one reaching the point they can challenge their Lord is too great. At the same time, he must keep them powerful enough that they can be an effective army to defend him should another Lord mount a challenge to place one of his own senior demons into Gil’s seat.
Gil’s view is largely, “If I keep my Demons happy, they’ll be more effective in defending me against challengers.” This has dual effects: the demons within his pit of hell are actually content and one could say they even enjoy their work. Unfortunately (for the souls of the damned, at least), when the Demons enjoy their work, their guests are all that much more brutalized.
While Gilrandree’s pit is traditional, others can vary greatly. There would certainly be others following more or less traditional design, but there would also be at least realms of bone-chilling cold, darkness and silence (until the beasts attack without warning), as well as one rich in foul substances, bodily waste, and so forth.
The pit itself is nothing more than an extension of the will of the Demon Lord in charge of it. As such, the personality of the ruler is strongly reflected in the pit. Gil, while often calm and in control, can burst out in horrific, brutal anger and violence at times, which is reflected in the fiery, intense nature of his pit. The pit characterized by great mounds and pools of bodily waste would be ruled over by a creature foul and generally unclean in every way, and so on.
In a very real way, the pit IS simply a manifestation of the Demon ruling over it, and within the pit, they are effectively all-powerful. They know everything which happens within it and can control the realm to the smallest detail. This power doesn’t extend outside of their realm, though, especially not into the mortal realm, though, where most people will encounter the Demon. If you're unlucky enough to encounter them in their pits, the old phrase very much applies: Abandon all hope, ye who enter here.