Firstborn

“	The ocean's first Seaborn, with body and power akin to a Feranmut. All the ocean's surviving Seaborn and Sea Terrors are His progeny.	”
	—Loading screen tip in Mizuki & Caerula Arbor

The Firstborn are four unique individual Leviathans who are the original source of the Seaborn.

Overview

The underwater ancient ruin recording the power of the Firstborn

The four emblems of the Firstborn: Growth, Survival, Reproduction, and Migration

Among all the Leviathans are four archaic individuals named the "Firstborn." They are the ancestors of all the sea monsters created in the ancient laboratories built by the previous civilization and wield magical power akin to the Feranmuts. The four of these Firstborn are:

Caerula Arbor/Izumik

Main article: Caerula Arbor

Representing the power of Growth, Caerula Arbor the "Creeping Branch" is the source of all Seaborn that have fallen to grace from the Many, hence becoming more of a food source for the Seaborn through Its corpse.

In an alternative timeline, the Caerula Arbor is resurrected as Izumik the "Source of Ecology" after Mizuki assimilates himself with Him.

Primordial Lifespring

The "Primordial Lifespring" represents the the power of Survival and is a swarm of plankton-like Seaborn cells born from all that remains of the namesake Leviathan that substance their existence. As It has merged with the ocean, the "Lifespring" is completely immortal to a point that, in a metaphorical sense, It could only be killed by "drying up the ocean." The "Lifespring" is said to be the source of the Profound Silence of Iberia as members of the Church of the Deep attempted to revive It.

Along with the "Corrupting Heart," the "Lifespring" is one of the potential Seaborn threats due to Its uncontrolled status, whose catastrophic consequences will be even worse. If merged with Ishar'mla, The "Lifespring" would bring a worldwide Silence across the entire Terra by flooding the whole world. It winning Its struggle against other Firstborn, It would even accelerate the assimilation of the whole living being of Terra to match Its desire.

Nevermelting Glacier

The "Nevermelting Glacier" represents the power of Reproduction and, true to Its title, possesses the shape of an enormous iceberg drifting on the sea. The Church's cultists see the "snow" above It possessing mystic symbolism, although unbeknownst to them, that "snow" could be a massive colony of Seaborn cells.

Ishar'mla

Main article: Ishar'mla

Representing the power of Migration, the "Corrupting Heart" is a devastating Firstborn named "Ishar'mla" whose rampage could bring disaster to the whole world.