The Ten Hells (Sometimes called Baator) are a group of linked demiplanes that contain a variety of landscapes and climates. The native species of The Ten Hells are devils. Quelmar had only Nine Hells for most of its history, until the Second Infernal Emergence in 792 PR led to Asmodeus' successful claim over Hellmar and the gods' recreation of Quelmar.
After the Second Infernal Emergence, Asmodeus cast a spell over Quelmar that prevented outsiders from teleporting deeper into the Hells unless they had previously traveled there before, or were using a stable and permanent portal. The longer a mortal remains in the Ten Hells, the more likely they are to be subjected to the effects of Infernal Corruption.
History
Before it was inhabited by devils, the realm that would become known as Baator was called Thalmyr and was ruled by the goddess of giants, Othea. With the blessing of the Creator Gods, Asmodeus killed Othea and transformed Thalmyr into Baator. Many of the first infernal inhabitants of Baator were those angels that aided Asmodeus in his murder of the goddess. This is the origins of the Hells according to the archdevil Mephistopheles and is recounted in his retelling of the the Pact Primeval.
The purpose of the Hells was to punish souls who defied the laws of the gods and to then use those souls to fight back the encroaching armies of demons from the Abyss. Originally, Baator consisted of only a single realm, but over time as Asmodeus grew in power, he pulled in other planes of existence, from elemental kingdoms to Abyssal wastelands to expand his infernal empire, turning them into new layers of the Hells. Asmodeus would always rule from the lowest plane of the Hells, his most recent prize, while putting the command of the other layers in control of powerful devils known as Archdukes. Each layer was connected through the River Styx.
Each layer of the Hells were inexorably tied to the Ruby Rod, the artifact that gave Asmodeus his power which he crafted from the heart of the fallen goddess Othea. Whoever controlled the Ruby Rod controlled the Hells, and vice versa. Each layer was also tied to a small piece of the Ruby Rod called a Ruby Heart. Just as the Ruby Rod itself could control all of the Hells, these Ruby Hearts could control their layers, and were bestowed by Asmodeus onto the layers Archduke.
The layers of Stygia and Cania were on the list of "possible convergence planes", noted for having the possibility of cross-contamination with eldritch powers of the Underdark Old Ones.
Baator's placement in the planes is right below the Underdark and Material Plane.
The First Infernal Emergence (181-246 BR)
After a failed ritual by drow mages in Pteris, a whole was ripped through the Material Plane, it the Underdark, and finally into the first layer of the Hells, Avernus. The Underdark's sea flooded into Avernus and devils were able to travel from Avernus now into the Material Plane. The flooding destroyed large portions of Avernus, and when the devils who traveled to Pteris returned, they rebuilt the first layer and named it Duumal.
The Second Infernal Emergence (792 PR)
Exploiting a loophole in the Pact Primeval, Asmodeus was able to drag the entirety of Quelmar and its citizen into the Hells and transform it into the Hell's tenth layer, Hellmar. While heroes were able to create a copy of Quelmar and forge a New Pact Primeval with Asmodeus, the original Quelmar still remained in the Hells. Asmodeus took up residence in Hellmar and his daughter Glasya became the new archduchess of the ninth layer of the Hells, Nessus.
The Infernal Invasion (904 PR)
Even after becoming the new Archduchess of Nessus, Glasya still maintained control over her previous domain of Malbolge. Though this weakened her control over both, and made both realms vulnerable. The straw that broke the camel's back involved experiments Glasya had been performing on Malbolge involving constructs, captured celestials, and portal magic. This weakened the interplanar barriers around Malbolge and allowed the creation of a portal between Malbolge and Duumal which allowed Demons to flood into Malbolge. In response to this failure, Asmodeus convened an infernal council to organize the military effort to reclaim Malbolge and reward the archdevil who was most accomplished during the effort with the position of ruler of that plane. Glasya was stripped of her title as ruler of Malbolge and was barred from participating in the competition, though was stilled required to aid in the slaying of demons. To prevent these demons from spreading deeper into the Hells, Asmodeus cast a spell over Malbolge that prevented creatures from entering or exiting the layer, with the only whole in this barrier being the portal between Duumal and Malbolge.
The winner of this competition would be the archdevil Philani and her draconic infernal consort Delphyne. They strategically positioned their legions to press in the demonic advance and used mortal adventurers to strike deep into abyssal territory. Eventually Philani and her forces were able to reach the portal and gain control of it, preventing any further abyssal invasions. For her success, Philani was made the new archduchess of Malbolge.
The Infernal Insurrection (925-XXX PR)
Layers
The First Layer, Duumal
Duumal is the top most layer of the Ten Hells. Originally known as Avernus, during the First Infernal Emergence the layer was flooded and destroyed, being rebuilt as Duumal, the new Infernal frontier, by the devils returning from Wild West Era Pteris. It is ruled by the archdevil sheriff of Duumal, Elytria, the fastest gun in the Hells.
The Second Layer, Dis
The layer of Dis is a barren and mountainous plane. It is home to the Iron City of Dis which takes up a majority of the layer. It is ruled by the powerful Archdevil Dispater. It is the largest hub for devils and weapon dealers across the lower planes.
The Third Layer, Minauros
The Third Layer of Baator, Minauros is an endless bog of vile pollution and decay. It is ruled over by the greedy and wealth Archdevil Mammon.
The Fourth Layer, Phlegethos
Phlegethos is a flaming hellscape of fire and magma. It is ruled by Fierna.
The Fifth Layer, Stygia
Stygia, the fifth layer of Baator, is a frozen wasteland. It is ruled by Belial the Devourer.
The Sixth layer, Malbolge
Malbolge serves as the prison for the Ten Hells. It was formerly ruled by Glasya who was the biological daughter of Asmodeus, until her promotion to ruler of Nessus. Callum was a barbed devil who served Glasya in her time as Malebolge's ruler.
The Seventh Layer, Maladomini
The Seventh Layer of Baator, Maladomini, is a layer filled with ruins. It is ruled by the ugly and disgusting slug lord Baalzebul.
The Eighth Layer, Cania
Cania is a layer of living cold. It is ruled by hells greatest wizard, Mephistopheles.
The Ninth Layer, Nessus
Nessus (also called Bolgia) is characterized by flaming landscapes and rivers of lava emerging from limitless plains of volcanoes. Jagged rocky peaks and steep drops into molten lakes provided a treacherous terrain for any creature without the ability to fly. For this reason, almost every resident of Nessus could fly safely around the flaming land. Asmodeus himself was the ruler of Nessus until the addition of the tenth layer, Hellmar. The Archdevil of Nessus sits on a throne in the middle of a towering, raging, fireball known as Malsheem. Malsheem contained millions of souls reaped from the Material plane in a labyrinth of chasms and peaks guarded by Asmodeus' best armies. After Asmodeus moved to rule the 10th layer, his daughter Glasya took his place as the ruler of Nessus.
Pathway to Hellmar
One gateway to Hellmar existed in the center of Malsheem. Guarding the gateway was the three headed dog known as Cerberus. The gateway itself required travelers to face their own fears, and was inscribed with the following poetry:
"To get below you'll show your worth. For if you want to see the earth Your darkest fears you must command do not run, you have to stand a chance to win and fears aswayed you'll soon Descend and go on your way."
Notable Locations
The Tenth Layer, Hellmar
Hellmar was the original iteration of Quelmar, given by the gods to Asmodeus per a loophole in the Pact Primeval. Though the gods had all but given up on Quelmar after Asmodeus took his claim, private consort with surviving members of the realm convinced the gods to "reboot" the Material Plane, allowing Asmodeus to continue his rule over the original with most mortals remaining blissfully unaware of any change. Since then the land has been twisted and corrupted into an infernal hellscape, and is the crown jewel of Asmodeus' empire.
New Citadel
Asmodeus, after leaving Nessus to rule Hellmar, built a massive citadel of magma and steel for his new seat as archdevil over the realm. The citadel was built alongside a new type of devil known as an Air Siren, who sing a horrid song so unholy that the air in the chamber itself flees, creating an unbreathable vacuum in whatever room the siren sings in.