The Sanctum Sanctorum was a network of bases scattered around the world by Sorcerer Supreme The Ancient One's order. There were three in all, located in London, UK, and New York City, USA.
The Sanctums were built in carefully chosen locations, atop major magical nexuses. The three Sanctums resonated with one another, forming a magical array that blocked the extradimensional Demon Gods from peering at Earth. They were also protected by a "Bystander's Veil," which kept ordinary people from seeing them unless they were mages or had been granted permission.
Aside from defending against extradimensional Demon Gods, the mages in charge of the Sanctums also monitored the flow of magic within their jurisdictions. This helped keep people who were curious about the mystic world from being misled by old books and ancient texts. Some merely recorded the bizarre imaginings of bored people from ages past; others might contain forbidden knowledge taught to highly gifted individuals by extradimensional Demon Gods through dreams. More dangerous still were the spellbooks deliberately cast onto Earth by the Demon Gods themselves.
The latter two were the most dangerous. People without formal training had no idea how powerful and perilous magic could be. If they rashly used spells from those old books or performed the rituals written in them, they were far more likely to bring tragedy upon themselves and those around them than to succeed. Some rituals could even tear open the barrier between Earth and the extradimensional realms, allowing the Demon Gods to reach their tendrils into the world.
Not everyone was like Hawking, who had glimpsed the universe's most fundamental and terrifying truths and emerged in one piece, gradually mastering the knowledge he had glimpsed. Even so, that knowledge had crushed Hawking's body, robbed him of his health, and imprisoned a giant's soul