The Sea God was a creature that lived by a small island that Guts' new party comes across as they travel to Skellig. It was sealed by the merrow long ago and worshipped by the island's inhabitants.
After the defeat of Ganishka and the emergence of the strange, white light enveloping the world, the Sea God grew in power and ate all the island's inhabitants except for Isma, who was protected by the charms she had received from her mother as a gift when she was younger.
Guts dons his Berserker armour with Schierke supporting his sanity and enters the Sea God's stomach through its mouth. He rides one of the several derelict ships up to a lining of the stomach and cuts a hole in it, and subsequently proceeds to exit the stomach via the monster's stomach gases. He then goes on to attack the heart, only to encounter giant insects and predatory fish. Guts deals with all of the lesser enemies but is stopped by the Sea God. The Sea God emits a sonic attack that causes Guts to bleed out from his eyes and his ears. Guts collapses and temporarily goes blind and deaf.
Helped by the merrow, who were attacking the monster from the outside with their magical singing, Guts eventually managed to stab the Sea God's heart after asking his inner demon to pierce his body in order to get him up and after getting directions from Schierke due to him being rendered blind from the sonic attack, but the gushing blood from the heart following the God's death was powerful enough to break the bond between Guts and Schierke, and the two are separated.
The witch abandons her astral form and comes back to the ship meanwhile Guts is trapped inside the Sea God's corpse, drowning in the beast's blood. The Sea God was not only immensely large and powerful; it could also sprout tentacles with which it could mimic the people it had eaten, similar to Apostle taking on the forms of their original humans.
The pirates eaten by the Sea God describe killing one of the tentacles as being akin to chopping off one of the Sea God's "beard hairs", a negligible loss by all means. Its defenses were not limited to its external body, as it contained several insects and predatory fish inside its body that defended its internal organs.
- Its heart has eyes all over it which makes its heart look a lot like the Idea of Evil.