p.s. the reason why I think it's about Seb-Cheneb is because in the preview at the end of episode 1, we see someone initiating a mind-meld with Jurati. We know from episode 3 that person is Commodore Oh, because when Jurati meets Oh her clothing and necklace match those in the mind-meld scene. This makes Oh a member of the Zhat Vash not Tal Shiar.
Other evidence for her, Narek, and his sister being Zhat Vash is that they're still probing Soji trying to find Maddox's lab, when it has already been destroyed by the Tal Shiar.
But, what is so darn profound/terrifying about Seb-Cheneb that it would bring down the submatrix of the Borg Cube upon them learning about it after assimilating Ramdha?
Well, we know from Hugh’s episode “I, Borg” that La Forge and Data postulated that an unsolvable problem might act like a virus to bring down the Borg, if correct then perhaps this secret might be something equally unsolvable or contradictory?
If it’s connected with Seb-Cheneb, then here’s a possible contradiction: the Borg are interested in assimilating new technology as a way of evolving and improving. But if they learn from Ramdha that this leads to the destruction of everything, then what’s the point of assimilation if Borg became all powerful and destroy everything including themselves?
Existence would be futile.