The Xa'a-ackétøth are a species of aquatic, serpentine creatures from somewhere deeper in the Milky Way. They were subjugated by the Affini Compact about 200 years prior to the Terran-Affini conflict. They do not have a large presence in prior Terran space as, being aquatic, they need different accommodations to most air-breathing species.
Their civilization was extremely warlike, focusing primarily on building vast arrays of autonomous weaponry, and by the time the Affini arrived they had been at a stalemate for some time. The array of weaponry—especially the war moons!—on display around Xa'at was sufficient to be a meaningful threat to the average Affini vessel. Despite this, at the point of first contract they accepted the Affini demand they submit to unqualified surrender, and were very grateful to find somebody who could prevent them from pointing semi-sentient weapons at their own heads.
They were domesticated about three or four species before the terrans and are incredibly violent! They'd almost wiped themselves out and had the kinds of weaponry that plausibly would've actually been a challenge to a non-special Affini cruiser! They never got to test it out tho 'cos they surrendered basically immediately and they're doing much better now! You don't see too many of them around, tho, cos they're aquatic and most of the ships around here aren't set up to provide aquatic creatures enough mobility to be attractive!
Physically, they have six eyes and a long, serpentine body.
See Divaricated Chapter Twelve for the initial mention and a description of what an affini from that area of space looks like.
Aquatic predators, the, ah... I don't think you'll be able to pronounce this one either, the Xa'a-ackétøth. A lovely species. They used to be very dangerous. They'd almost destroyed themselves and their many worlds by the time we came across them. Surprising even us, they shut down their war engines and surrendered immediately as soon as we offered them a place under our wing, metaphorically speaking.