The Center for Reunification was a building used as a cult that was run by Baron Mordo.
Baron Mordo set up a cult in a mansion that he called the Center for Reunification.
Mordo made the cultist believe that the purpose of the Center for Reunification was to reunite them with lost loved ones. However, the truth was that Baron Mordo was using his magic to make the cultist believe that they were meeting long lost loved ones which in turn allowed Mordo to mentally control the cultist. This was done so that Mordo could free his master, Dormammu.
After several cultist attempted to steal the Wand of Watoomb from the Sanctum Sanctorum, Spider-Man followed them to the Center for Reunification. However, Spider-Man was unable to get into the Center for Reunification because it was protected by the Crimson Bands of Cyttorak.
Peter Parker later posed as one of the cultist and got inside the Center for Reunification to rescue Mary Jane Watson who had been brainwashed by Baron Mordo. However, Peter was captured and brainwashed by Baron Mordo. Spider-Man then went to the Sanctum Sanctorum and helped the cultist steal the Wand of Watoomb. However, Doctor Strange was able to free Spider-Man from Mordo's mental control.
Spider-Man, Doctor Strange, and Wong then went to the Center for Reunification and Doctor Strange used a spell to open a hole in the Crimson Bands of Cyttorak and went inside. Inside the building Doctor Strange fought Baron Mordo. After defeating Mordo, Doctor Strange used the Wand of Watoomb to close the portal. However, the rapid decrease of magical energy caused the Center for Reunification to fall apart. Doctor Strange then teleported himself, Wong, Spider-Man, and Mary Jane outside and watched as the Center for Reunification disappeared into thin air.