The Meeting in the Elder's Chamber is where Numbers Four, Five, Six, Seven, Nine, as well as Lexa, Adam, Setrákus Ra, and all Human-Garde (hundreds) meet to start rallying against the Mogadorians and to end Setrákus Ra.
After Ella is possessed by the Lorien entity (whom she calls "Legacy"), she uses her amplified telepathic powers to gather all who have been touched by Legacy, including but not limited to all Loric Garde, Setrákus Ra, Adam, Lexa, Sam Goode, Daniela Morales, Bertrand, the U.S. President's older daughter, a boy from London, a girl from Japan, a boy from Nigeria, and hundreds more.
Pittacus Lore and Setrákus Ra were friends growing up, but Setrákus Ra was obsessed with discovering how to harness Lorien's power with his Pipeline prototype to grant powers to those who are not Garde like his wife, Celwe. Pittacus was going to have executed Setrákus Ra at the orders of the other Elders but became merciful instead and exiled him. Years later, Celwe came back to Lorien with her daughter, Parrwyn, who became Ella's grandmother.
When John and Nine are sent to the Elder's Chamber, they see Five, and Nine attacks him to no avail. The Loric Garde and Adam all see their fallen Cêpans and allies (Four sees Henri, Nine sees Sandor, Five sees Rey, Six sees Katarina, Marina sees Adelina, and Adam sees Number One). The Cêpans escort the Garde to the Elder's chamber, where they meet Lexa, Ella, Ra, and all Human-Garde.