Let me be the first to say what is on all of our minds:
Sorcerous Phalli.
But seriously... good question, FB.
I'd gather that they either had preexisting knowledge about the so-called "Promised Ground" and sorcery's existence on Earwa allowed them to run a bunch of previously programmed grafting trials that might work only on Earwa or still had enough knowledge of the Tekne collectively at that point to actively figure out what makes the brain/body "see" the onta (they do most of their thinkenearing with the Tekne over the latter part of Cu'jara Cinmoi's life).
As per
Pat's Part 2, 2011 interview and the
Cuno-Inchoroi Wars entry of TTT Glossary:
- There is no evidence of the surviving Inchoroi using sorcery during the two major battles with the Nonmen (or in the Glossary's telling of the Cuno Inchoroi Wars:
- Battle of Pir Pahal (first conflict): Nonmen of Viri and Inchoroi turn on each other, the hosts of Cu'jara Cinmoi save some of the Virian Nonmen, and the Inchoroi are beaten back to the Ark.
- Battle of Inniur-Shigogli (Post-Womb Plague): inside the Ring Mountains raised by the Ark's Fall; weapons races are unleashed, surviving Inchoroi still using weapons of light and protected by Chorae (which actually doesn't give us an indication as to whether or not the Inchoroi have already attempted the graft).
However, major clues from the interview:
The Inchoroi only possessed the Tekne when they arrived in Eärwa. All of the Inchoroi are the products of successive Graftings, species-wide rewrites of their genotype, meant to enhance various abilities and capacities, such as the ability to elicit certain sexual responses from their victims (via pheromone locks), or the capacity to ‘tune sensations’ and so explore the vagaries and vicissitudes of carnal pleasure. The addition of anthropomorphic vocal apparatuses is perhaps the most famous of these enhancements.
The Grafting that produced Aurang and Aurax was also devised during the age-long Cûno-Inchoroi Wars, one of many failed attempts to biologically redesign themselves to overcome the Nonmen. But they had been outrun by their debauchery by this time, and had lost any comprehensive understanding of the Tekne. The Graftings had become a matter of guesswork, more likely to kill than enhance those who received them. The Inchoroi filled the Wells of the Aborted with their own in those days.
Aurang and Aurax are two of six who survived the attempt to Graft the ability to see the onta.
- only possessed Tekne when they arrived in Earwa.
- successive Graftings, species-wide rewrites of their genotype.
- during the Cuno-Inchoroi Wars.
- two of six who survived the attempt to Graft the ability to see the onta.
There are five hundred years after the Battle of Inniur-Shigogli of Cuno-Inchoroi Wars before the Nonmen trap the Inchoroi in the Ark and then another twenty years fighting inside before Nil'giccas instructs the Artisan and other Quya to fashion the Barricades (thinking they got them all?).
I'd say, considering the above points that each successive Grafting is a species-wide rewrite and that two of six survived, this must have been a late-game Hail Mary, probably during those twenty years fighting inside the labyrinthine Ark...
EDIT: I really didn't do your question justice, FB. Maybe it's like Bakker's Blind Mary/Red Argument: the ability to see the onta requires a specific neural architectural pattern?