The Sanctuary of the Clans was the Council chamber and Throne room of Kain's empire; it was built around the remains of the Pillars of Nosgoth with the Pillar of Balance as the base of Kain's throne, and was seen in Soul Reaver.
The Sanctuary of the Clans was first seen in the Soul Reaver opening video; it was the location of the council meeting where Kain ripped off Raziel 's wings and ordered his execution. Raziel would return five centuries later following his execution into the era of Kain's empire, shortly after emerging from the Underworld . He was unable to gain entry to the Sanctuary as the doors and gates were rusted shut, so he continued to the nearby Abyss and onto his Clan territory.
After defeating Melchiah and gaining the Phase Through Gates ability, Raziel was able to enter and found Kain by his throne, apparently awaiting Raziel's return. Raziel confronted Kain and Kain responded by drawing the Reaver blade from its scabbard, after a short battle Kain defeated Raziel and smashed the blade against Raziel, shattering it. Kain was seemingly expecting this outcome and teleported away his laughter ringing out through the Stronghold. In the Spectral Realm Raziel found the spirit of the blade now in its natural form - the wraith blade. During this visit he was also able to find the bound spirit Ariel, who would offer further guidance whenever it was required.
Raziel visited the Sanctuary of the Clans at least once more, shortly after his defeat of Zephon, where his new 'Scale Walls' ability enabled him to climb to the upper balcony and use and exit on his way to the long-hidden Tomb of the Sarafan.
The Sanctuary Of The Clans appears to be much bigger and brighter in its presentation in the opening video of Soul Reaver than in its playable counterpart.
Considering the front gate is rusted shut, it can be assumed that the Sanctuary has not seen much in the centuries since Raziel's execution; it is slowly crumbling away as with the rest of Kain's empire by the time of Soul Reaver.
Kain is also observed briefly in a similar room at the opening 'dream' sequence video of Blood Omen 2, although this is not actually The Sanctuary of The Clans. Whether the Sanctuary of the Clans has been built by this stage is unknown, though the Pillars are one of the locations that Kain's vampire army conquers, it is unlikely they had time to do any major building before the army was destroyed by the Sarafan Lord.