The Seaborn are hyper-adaptive aquatic creatures hailing from the abyss of Terra's oceans. They evolve incredibly quickly, possessing the ability to mutate their own somatic cells without relying on generational heredity. Unlike the Aegir race that also live in the oceans, the Seaborn possess a hive mind with a collective memory and have no sense of individuality. Because of their adaptivity, they are constantly absorbing new information from Terrans and using it to further their own evolutionary progress. They display a wide variety of body types, ranging from plantlike to quadrupedal to fully humanoid.
There are different classifications of the Seaborn. The Sea Terrors are considered the lowest-class creatures that behave like normal animals. As they evolve, those who start to develop more human-like intelligence and consciousness transform both physically and mentally. These more sophisticated Seaborn are responsible for spreading their knowledge to their kin. Despite their differences in biology, all Seaborn are treated as equal and have never been observed fighting one another for food. It is also nearly impossible to study a living Seaborn specimen in a research setting as they are able to reduce their bodies to a mass of proteins when being captured.
Seaborn have unique characteristics. One is their incapability of telling lies and their lack of understanding on emotions. The First to Talk, the first Seaborn to communicate with the Terrans, expressed confusion at many concepts such as dishonesty and hatred. Additionally, they are extremely pliable and will act on suggestions without much deeper individualistic consideration. The other is their craving for assimilation of other creatures driven by their hive minds, as in the case of Skadi the Corrupting Heart, but this is also possible to resist this corrupting influence with enough willpower. As they value unification and assimilation , they are capable of forming transactional symbiotic relationships with human communities, as seen in Sal Viento, where they traded food for new members of their horde. Despite the Church of the Deep's worship of them, the Seaborn do not and will never give them special treatment.
Historically, the Seaborn have been involved in a long-standing conflict with Aegir and Iberia, which partially arose as a result of Aegir colonizing their territory and the Iberians' attempt to explore the oceans. In an effort to reclaim their land, the Seaborn attacked infringing Aegirian settlements. In the meantime, they are also responsible for the Silence of Iberia that forced the country to isolate itself. Until present days, the Iberian government stubbornly refuses to reveal the existence of the Seaborn to Terra's land-dwellers.
As an effort for retaliation, Aegir began to experiment with different methods of eliminating them which lead to the creation of the Abyssal Hunters, a band of Aegir-Seaborn hybrid supersoldiers. Because they are infused with Seaborn blood, all Abyssal Hunters are viewed as brothers and sisters by the Seaborn who harbor no ill will towards them. However, despite this, their hostility is noted by the Seaborn, who will attack in retaliation when attacked by a hunter. Less intelligent Seaborn mistake the Aegirian bodies of Abyssal Hunters to be prisons for their kin, thus attacking them in an attempt to free the blood within.
The origins of the Seaborn can be traced back to their monstrous "genesezoic" Behemoths. One of them, named "Ishar-mla", is believed to have been killed in a full-scale assault by the entire Abyssal Hunter forces, upon which Skadi dealt the final blow. However, it seems not only this creature was not killed, but also it actually merged with Skadi and has been falling into a long, deep sleep. To make the matter worse, there are numerous genesezoic Behemoths hibernating within the ancient relics beneath the oceans. If awakened, they could bring another disaster not only to Aegir but to the surface of Terra.
- Rocinante: A horse-like Seaborn acting as the Last Knight's mount. Its sheer size allows its to control the waves.
Of the above characters:
- Gladiia, Skadi, Specter, and Ulpian are not "true" Seaborn, but Aegir-Seaborn hybrids created by Aegir as part of the Abyssal Hunter project.
- Skadi the Corrupting Heart is an alternate timeline version of Skadi where her Seaborn blood has overtaken her and caused her to assimilate into the hive mind.
- Specter the Unchained remains an Aegir-Seaborn hybrid, but she has resisted her insanity caused by her Oripathy through her Seaborn blood.
- Alfonso is partially a Forte and a Seaborn.
- Amaia is formerly a Liberi; she was transformed into a Seaborn after willingly assimilating with Assimilator.
- While Mizuki is technically an Aegir, it is implied that he had been fed with a mysterious object by a follower of the Church of the Deep which infused him with Seaborn blood.
- Quintus is formerly an Aegir; he transformed into a Seaborn through some unknown causes.
- The Last Knight is formerly a Kuranta.