SCREENRANT VIDEO OF THE DAY SCROLL TO CONTINUE WITH CONTENT Summary Star Trek: Discovery may have just secretly introduced an unseen alien race from Star Trek: Deep Space Nine, ahead of their mission to Tzenkethi space. The Tzenkethi were first mentioned in DS9's season 3 finale, "The Adversary", in which a Changeling impersonator attempted to trigger a war between the Federation and the unseen alien race.
If there is a Tzenkethi diplomat aboard Federation Headquarters in Star Trek: Discovery season 5, it will make things easier for negotiating entry into their territory. However, the very fact that entrance into Tzenkethi has to be negotiated suggests that, even centuries after their war with the Federation, they've never become members of the UFP.