The Conclave of Eight was an organization mentioned by Raffi Musiker in her conspiracy theories about the attack on Mars. (PIC: "Stardust City Rag")
She originally thought it was named for the people involved in the attack, but after learning about an octonary star system in old Romulan star charts and the Zhat Vash, she began to think it might be named after the group's meeting place.
On Aia in the 2380s, Narissa, after her experience with the Admonition, asked Oh how they were going to stop artificial lifeforms from destroying all life. Oh replied "On the world the Humans call Mars." Narissa then told her aunt Ramdha that they had work to do. (PIC: "Broken Pieces")
The Zhat Vash were the real group that planned the attack. The precise relationship between the Zhat Vash and the Conclave of Eight is unclear.