Avernus Cathedral first appeared in Blood Omen, and it was located in the centre of Avernus. It was presumably the focal point for the citizens' unspecified religious beliefs.
Kain visited Avernus Cathedral in Blood Omen in order to kill Azimuth the Planer. When she decided to destroy the city of Avernus, the demons she summoned left the cathedral untouched, knowing "better than to bite the hand that feeds them". Ariel gave Kain some enigmatic information regarding the Planer's home. She told him:
"Three instruments await you, to aid you in your quest, but first you must rise and you must fall and find your salvation in between."
The "three instruments" were the Soul Reaver, the Wraith Armor and a Time Streaming device. Avernus Cathedral held them all, but it was no ordinary building. Azimuth was the Guardian of the Pillar of Dimension, and the cathedral reflected this. There were lots of teleporters, and it was split into three very different environments. Kain had to 'rise' into a heavenly area to collect the Soul Reaver, and 'fall' into a hellish area (the Avernus catacombs) to retrieve the Wraith Armour. In between, in the 'normal' part of the cathedral, Kain killed Azimuth and picked up the Time Streaming device. This later allowed him to escape from the Battle of the Last Stand.
In Blood Omen, Avernus Cathedral was one of the most interesting places in Nosgoth. Its contents had great historical significance. In addition to its contrasting levels and the items already mentioned, it housed an impressive stained glass ceiling mural of Vorador and Malek. Also, in the depths of the hellish catacombs, there was a skeleton, suspended by chains. The stripped corpse was likely the mortal remains of Malek, left there from his damnation around five hundred years earlier. It certainly looked like it, despite Malek claiming a corpse in his Bastion was his. Prima's Official Blood Omen Strategy Guide actually refers to both sets of remains as Malek's.
Last of all, there was a secret area that gave away the true nature of the cathedral, exposing its public appearance as a façade that hid a darker form of worship. A second religion was observed within the cathedral: one that appeased a god's demands for blood sacrifices. Kain could find an altar dedicated to this god - Hash'ak'gik - in an upper chamber of the Avernus catacombs (see the encyclopaedia entry for Hash'ak'gik's Altar for more about this).
Avernus Cathedral appeared again in Defiance, and although it was a very important location in that game, only a small part of it was explored this time. It also looked quite different from its Blood Omen incarnation. Raziel travelled to Avernus Cathedral to try to find the Heart of Darkness; Vorador had directed him there, believing the heart had been taken to the cathedral as a trophy. The old vampire warned Raziel of "dark sorceries at work in the cathedral" and "an ancient evil" within, "long Unspoken among [Vampire] kind." While Raziel was there, he found the Warp Gate to the Earth Forge and imbued the wraith-blade with elemental Earth. Afterwards, he managed to open a portal to the Avernus catacombs.
Raziel entered the catacombs and found murals that elaborated on the Hylden/Vampire history he already knew. However, these had apparently been painted by the Hylden, and showed things from their perspective: "how the noble Vampires, god-ridden and righteous, had started the wars that would destroy both races, victor and vanquished alike." Aided by the Elder God's misdirection, Raziel came to the conclusion that he was the Hylden champion, destined to destroy the Vampire champion.
At the end of the catacombs route, Raziel opened some double-doors. Had he been a little more observant, the symbol they bore would have given him a clue to what awaited him beyond. They were marked with Turel's clan symbol (as seen in Soul Reaver).
Raziel stumbled upon Mortanius leading the Avernus Cenobites in their worship of Hash'ak'gik. Raziel wondered if this was the 'Unspoken' Vorador had mentioned, but it seemed more likely that Vorador was referring to the general Hylden influence within the cathedral. The presence of the murals (already mentioned) hinted that the Avernus catacombs may have once been a Hylden stronghold. As Raziel soon found out, they certainly had a foothold in Avernus at the present time - the Blood Omen era. The 'god' Hash'ak'gik was in fact the evolved vampire Turel, intermittently possessed by several Hylden. He liked to think of himself as a god - "greater even than Kain" - but it was their words and will that the worshippers responded to. Turel was only a figurehead, brought into the past by Azimuth¹, and a prisoner within a pit, surrounded by the skeletal remains of his countless victims.
Raziel killed Turel, and had the opportunity to speak with Mortanius. During their conversation, he learned that the Heart of Darkness was inside Kain: Mortanius had used it to resurrect the human Kain as a vampire, after orchestrating his assassination in Ziegsturhl. Mortanius - misunderstanding the ancient prophecies like nearly everyone else in Nosgoth - believed that Kain, the Scion of Balance, was the Vampire champion destined to destroy Raziel, whom he thought was the Hylden champion. Kain was not the Vampire champion at all; actually Raziel was both champions depicted in the murals, Hylden and Vampire.¹
As Raziel left the catacombs he ran into Kain, and their inevitable confrontation occurred. Kain was adamant that the heart should remain undiscovered, despite having no idea where it was. Raziel was victorious in their fight. He thrust his claws into Kain's chest and wrenched the Heart of Darkness out. Kain was thrown through a portal by Raziel's telekinesis, but Raziel was sure that wherever his body was, Kain was certainly dead. Raziel believed that the "madness" of Avernus Cathedral had somehow fuelled his rage against Kain.
Against the expectations of Raziel, Kain survived. He awoke in Turel's pit, in the demon dimension/demon realm. Some Hylden spoke to him while he was there, promising that the demon realm would render him hideous and mad. Kain was not trapped there in the way that they were, however. Once he had dispatched a few demons, a portal opened that allowed him to exit the demon dimension. Soon after, he departed from Avernus Cathedral, heading for the Vampire Citadel.
We have never been given details of the official religion of Avernus, but there are a few clues. As Blood Omen Nosgoth seemed rather medieval-european in terms of its society, the cathedral probably promoted a male-oriented monotheistic faith. On the evidence provided, we can't clarify much beyond that, except that it incorporated ideas of heaven and hell, and angelic symbolism. The Sarafan - as warrior and sorcerer-priests - appeared to acknowledge the same religion in waging their 'holy war' against the vampires. Based on the later games in the Legacy of Kain series, we can see where the angel imagery likely originated from, and it's possible that this unknown religion evolved from the Ancient Vampires' worship of the Elder God.