This article contains major spoilers for Zack Snyder's Justice League. You have been warned.
Originally part of a larger Atlantis, they broke from the main kingdom following the Great Fall and formed their own breakaway faction. They traditionally hold animosity with the Kingdom of Atlantis, due to their rivaling military power.
Becoming a subspecies
After the sinking of Atlantis, the people evolved and developed into their own subspecies, such as the Xebelians. The Xebelians and the Atlanteans are the only humanoid species of Atlantis and the only two species to remain fully human in appearance, while the Fishermen evolved to fish-like humanoids, the Brine evolved into crab-like creatures, and the Trench regressed into savage creatures incapable of speech or intelligent thought. The Kingdom of the Deserters died out as the Sahara became a desert and the Missing Kingdom was reported to have completely vanish without a trace.
Between the 1980s and the 1990s, a series of battles involving the Xebelians occurred. While it is unclear who exactly they fought, it is known it wasn't Atlantis, as the Atlantean royal family took the Xebelian princess Mera in while her parents, the Xebelian king and queen, fought in the wars.
The Xebellians and all the other unified kingdoms warred against the Kingdom of the Brine, suffering many casualties as a result. As the battle raged on, Arthur interrupted the battle and ultimately destroyed several Xebellian warships with aid from the Karathen. After Orm was dethroned, the Xebellians surrendered and acknowledged Arthur as their new king.
Xebelians appear to be physiologically identical to Atlanteans.