In the course of his quest in Blood Omen Kain visited Avernus and it's Cathedral to challenge the Guardian of Dimension - Azimuth the Planar. Beforehand he was advised before by Ariel that he would have to "rise and fall and find [his] salvation in between" When Kain arrived in Avernus Cathedral he found it was apparently split into three 'realms', with teleporters linking them; these were seen as the Normal Cathedral; a bright white 'heavenly' realm; and dark 'hell' realm. The 'hell realm' was later revealed to be the Avernus Catacombs.
In the Catacombs the young Kain was able to find the Wraith Armor and several other interesting areas including one that resembled a scene in the opening FMV and appeared to contain Malek's corpse, as well as the hidden Hash'ak'gik's altar, which showed a secondary religion that was active in Avernus which demanded blood sacrifices.
The Catacombs were revisited in Defiance, apparently a short while after the young Kain had been through.Raziel, coming to the Cathedral in search of the Heart of Darkness was able to gain entry to the Earth Forge (which was apparently also underneath the Cathedral) and imbue the Earth Reaver . This enabled him to climb high in the Cathedral to pick up the 'Dark Scripture' which allowed him to enter, through a portal, 'Hell Realm' of the Avernus Catacombs.
In the Catacombs, assailed by Cenobites who summoned Red Fire Demons, Raziel discovered murals of the Elder Wars, except this time they were from the Hylden perspective and they showed the Ancient Vampires as terrifying aggressors, descending from the skies to attack their enemies for refusing "to submit to the Wheel of Fate ". He also found an image of how the Hylden saw their own champion - a dark silhouette in the shape of Raziel, whose flaming sword now seemed much more like the Wraith-blade . This champion was to "destroy the shackles of the vampires tyrannous god ."
Further along the chambers he found his way to a large pit where Mortanius and the Cenobites were apparently worshipping the Hash'ak'gik entity. When they had finished Raziel moved close enough to the pit to be telekinetically dragged into it by the Entity, which was revealed to be his (d)evolved brother from the far future, Turel, who was possessed by several Hylden. Battling Turel, Raziel was able to devour his soul and finally gain the Amplified Force Projectile (that was removed from Legacy of Kain: Soul Reaver along with Turel) which brought his telekinetic abilities up to Kain's standard.
Exiting the Pit, Raziel met the Mortanius, (who was leaving a message for the younger Kain.) Mortanius, much like Truel was possessed by Hylden, this time a single Hylden - the Hylden Lord, who confired that it was he who put the chain of events that led to the corruption in motion by murdering Ariel. Raziel also got Mortanius to admit to hiding the Heart of Darkness inside Kain. As Raziel exited the Catacombs to the Cathedral, Kain entered and the two battled. Raziel won the battle, claiming the Heart of Darkness and sending the apparently dying Kain through the portal back to the Catacombs.
Some time later Kain woke in the Demon Realm and found himself in Hash'ak'gik's pit. Attacked by a 'demon horde' Kain was able to fight them off and make his way back to the Material Realm through the portal. Shortly afterward he left the Catacombs and Avernus itself and returned to the Vampire Citadel -apparently feeling the pull of the Spirit Forge .
The Catacombs are another area that changes radically between its appearances in Blood Omen and Defiance. In Blood Omen, the walls are piled high with bones and there are many skeletons and other supernatural creatures in the Catacombs. In Defiance the skeletons are gone, replaced by Cenobites and Demons (which did not enter the Cathedral in Blood Omen) and the bones are seen to be merely bone-like ribbed walls.
The chained corpse seen in Blood Omen, although not identified as such, certainly resembles the corpse of Malek seen in the opening video and is perhaps a better candidate that the one identified by Malek in his bastion .
In Blood Omen, Hash'ak'gik's Altar has on it a bloodstained book (which may be related to the Dark Scripture seen to open the way to the Catacombs in Defiance.)The book has in it the text 'And Hash'ak'gik spoke to the world and all who heard trembled "bring me your first born and shed their blood on the altar of the world so I might take nourishment from them, do this without question or suffer my wrath for eternity." and its will was done' The passage certainly bears a resemblance to the Kain's execution of Raziel and may be indicative of Turel's witnessing of the event. Further clues to Turel becoming the Hash'ak'gik entity can be seen on the doors to the Pit Chamber in Defiance, which are marked with Turel's clan symbol from Soul Reaver.
Azimuth is, although absent in Defiance, (she has already been killed by the younger Kain) highly implicated in the events there. She was likely to have been aware of and been a leading figure in the Hash'ak'gik Cult and arguably may have had more knowledge of the true face of the Cult, as she was Dimension Guardian. In Blood Omen she carried with her a Time Streaming Device, which she had planned to use to gather Demons from other times. It now seems she managed to use this device to bring Turel to Avernus from the far future, which also means she would have been aware that Turel was merely an impressive front to encourage worship of Hash'ak'gik amongst the Guardians, making them vulnerable to the Hylden's plans, indeed Mortanius still wasn't aware of the true face of the cult in Defiance.