The Valtheri Sequence-XX Extinction is an ongoing event that began during the Watershed in 4100 CGC with the attempted slaying of the verge god known as 48-5: Pursuer. Since this verge god's seeming "partial death," the valtheri species have not had any new individuals with XX chromosomes. Some individuals may be born with XXY chromosomes, but they are infertile in virtually every known case and are rare throughout Ealdremen besides. Because the valtheri naturally rely on sexual reproduction between individuals with XX and XY chromosomes to populate, this has caused the species to have a noticeable drop in numbers. Valtheri with XX chromosomes have not even been producible in laboratory settings with surrogate pregnancies; the second X chromosome will always morph into a Y chromosome early on in pregnancy, or the resulting fetus will be not survive to term.
Hormonal treatments and surgical transplants have not successfully "replaced" the need for XX chromosomes in the valtheri. Although this enables a given valtheri to give birth, it does not produce eggs necessary for conventional sexual reproduction. Genetic material from other parts of a valtheri's body, such as bone marrow, have also not been successful in replacement and providing a sustainable future for the species.
Traditional valtheri beliefs point to the partial death of 48-5: Pursuer, which they know as the deity "Theriasor," as the reason for this decline of their species — the "part" of the god that died was the one that created this portion of the species, and without this part of 48-5: Pursuer, the valtheri population corresponding to it will permanently cease to exist. While aspects of this narrative are questioned — living valtheri with XX chromosomes did not abruptly die out all at once and had a steady decline — it has proven to be a point of research into numina corresponding to 48-5: Pursuer to understand if there truly is a connection between the god and what has become of the valtheri.
Although modern understanding points to the event beginning with the Watershed, it took years for data to be aggregated enough that it was an observable phenomenon across all documented valtheri births. As time went on, remaining individuals with XX chromosomes began to die out from normal causes, but were never replaced by new generations. As of 4364 CGC, the valtheri have no living individuals with XX chromosomes, and the species is sustained through dwindling repositories of frozen eggs from XX donors that lived centuries ago and cloning of existing individuals. Typically, fertilized eggs are carried to term by surrogates of another related species, most commonly sigkronakkos, though surrogate kronakko and gujavn pregnancies have been successful, and it is theorized that a medrin-born surrogate pregnancy could be possible. Valtheri who have the necessary hormonal treatment and surgery are able to carry a child to term in this same manner.