"So we would like to invite you to join us in our secret expedition."
Apparently, The Emperor who also has visited the Six Path Realm expressed his desire to go there once again upon realizing just how much valuable resources other two empires get thanks to those violet portals.
That's why, the Cadere Clan which was first clan to offer its services began preparing for secret infiltration. They had thoroughly investigated the violet portals that had appeared in their territories in the past, thus, they knew about purpose of those mysterious portals.
But that's pretty much all they could get other than the artifacts from this very mysterious realm.