Avernus is the first plane of the Nine Hells. The uppermost plane of the Nine Hells is ruled by Tiamat, the Chromatic Dragon, and serves as a home to all lesser unique devils. These are devils with individual names and characteristics, and powers of greater magnitude than those of a pit fiend (for all such unique devils of lesser power are soon destroyed by their enemies and the cruelty of their fellows), which are not otherwise placed in the hells as ruling archdevils or their direct servants.
There are approximately fifty of these lesser unique devils; embittered, frustrated beings who torment the lemures and spined devils of this plane and viciously attack all intruders. Chief among them is Nergal, who like all of his outcast fellows, plots and schemes constantly to win a higher rank, but through hopes of reward always eagerly and enthusiastically obeys commands issued by the archdevils. Most such commands concern the defense of the hells against all intruders, for the dukes of hell have no wish to deal with intruders on their own ground – the plane or planes they rule – when such disorderly business can be conducted elsewhere. The uppermost plane of the hells thus serves as a marshalling area and training ground for infernal armies and is (preferably to the devils) the place in which intruders are dealt with.
It is a plane of darkness, consisting of rocky crags and hillocks, a desolate wasteland of stunted, poisonous vegetation and bare rock. Many concealed pits lined with filth-smeared stakes and the like have been prepared for intruders. Avernus has no roads or buildings, but many caves and warrens have been dug out of the rocks; most are devils’ homes, as unwary travelers seeking shelter will soon learn.
At irregular intervals the darkness is lit by fireballs that form spontaneously from the vapors of the air, bursting at any height above the ground. If one should notice a glowing, swirling effect in the air, one soon discovers that a fireball will burst in that location one round later. The precise cause of these fiery discharges is unknown, but it is natural and not under the control of any devil or another creature.
Any non-devil lawful evil creatures not native to the hells (such as the occasional beholder), if they are to be found in the infernal regions at all, will be found on Avernus unless specifically located elsewhere by order of an archdevil. One noteworthy example of such creatures are the nycadaemons. These have the power to move freely about the Nine Hells, but they are disliked and distrusted by the archdevils and may well be met with physical opposition if they appear uninvited on a plane other than Avernus.
Avernus is nominally ruled by Tiamat, the Chromatic Dragon. The mother of all evil dragonkind holds Avernus through the support of Asmodeus, but her actual authority extends only as far as her physical reach. Most of the inhabitants of the plane avoid her, not wishing to serve as her meals.
The Chromatic Dragon spends vast majority of her time sprawled in her lair, the great caverns known as Azharul, “The Dragonspawn Pits.” Here she is attended by her bodyguards, five huge adult male dragons – one white, one black, one green, one blue, and one red. She is constantly guarded by these consorts, and when not plotting, feeding, enjoying cruel sport or facing danger, Tiamat will be found mating with one of them within a protective ring formed by the other four. She bears litters of up to four dragonets (or “mewlings”: very young dragons born conscious and in control of their feeble powers) after an average gestation period of 6 days.
Tiamat occasionally travels via the Astral Plane or Ethereal Plane, perhaps to give birth to a litter on the Prime Material Plane (or Planes, if the multiverse of your campaign includes “parallel worlds”) – sometimes grudgingly, at the behest of Asmodeus, but more often to further some plan of her own. The scant remainder of her time is spent roaming Avernus; very rarely, she journeys to the palace of Asmodeus. Those of Tiamat’s offspring that are born and remain on Avernus occupy themselves with hunting down and bringing back food for Tiamat and her consorts while the Chromatic Dragon is in her lair. These offspring/underlings are of all sizes, types, and ages of evil dragonkind, and all are aggressive, cruel, and in good health. Injured, weak, or disobedient specimens are soon eaten by Tiamat, or by others at her direction; she also dines on slain dragons, including slain consorts who have displeased her, and all newborn spawn who are multi-headed or otherwise chromatic in nature. The few of her spawn that survive to achieve “huge adult” status serve as replacements for her consorts.
Tiamat will have full amounts of treasure only while in her lair. She may seize small amounts of treasure when on the Prime Material Plane, but only quantities which she can transport herself, since her bodyguards are unable to accompany her when she leaves Avernus. They guard her hoard in Azharul diligently in her absence, for she will notice if even a single bauble is gone! The treasure prized most highly by Tiamat is magic – specifically, Wizard spells of 1st to 5th level which are new to her. With these she can increase her personal power, and consequently she is always seeking more powerful offensive and defensive spells which she can study, experiment with, and modify for her own use.
Although Tiamat will battle (attempting to kill and eat) individual devils without hesitation, she will never knowingly attack an archdevil or a lieutenant to an archdevil. She rules her spawn by force and fear, but her will is enforced on Avernus by 40 companies of abishai under the command of Malphas, 29 similar companies led by Amduscius, and 3 companies of erinyes under the command of the pit fiend Goap.
But Tiamat is just one of in the Nine Hells, and not the most powerful, so she doesn’t want to bring too much attention to her lair, else she could find herself as an outcast. Among these outcast devils are names such as Rumjal, Caim, Bist, Cahor, Dagon, Azazel, Armaros, Kochbiel, Nisroch, and Nergal.
Tales are told of a mysterious archdevil, Astaroth, said to be mightier in power than any devil save Asmodeus himself who dwells elsewhere than the Nine Hells, apart from the present regime, working alone to advance his secret ends. This devil represents the greatest threat to Asmodeus; with Astaroth’s support, Mephistopheles, Baalzebul, or even a lesser archdevil such as Geryon could conceivably wrest control of the Nine Hells from the Great Devil himself. But even the existence of Astaroth is uncertain; he is definitely not an avowed force in the hells at this time.