Poseidon's change of heart and her eventual alignment with the Demon Lord was not a direct consequence of demonic energy corruption leading to monsterization, but rather a conscious decision to defect. Poseidon grew disillusioned as a result of the Chief God's orders to kill humans with storms and tsunamis and the Chief God killing sea monsters and devotees of the sea god. Since her defection, being dragged under the waves no longer ends in death by drowning: Poseidon's magic monsterizes women into nereids, and sustains men until one of her devotees arrives to rescue him. The sea god still calls down great storms to shipwreck sailors, but does so in order to provide male husbands for the monster girls of the sea.
During the age of the former Demon Lord, Poseidon was heavily involved in a great war between the forces of Heaven and the demons of chaos. These eldritch demons were sealed in the deepest part of the sea, and even today, thousands of years later, this abyssal zone lies beyond reach of Poseidon's influence.
During the age of the former Demon Lord, Poseidon was in the vanguard of a great war between Heaven and the demons of chaos. These eldritch demons were sealed in the deepest part of the sea, and even today the abyssal zone lies beyond the reach of Poseidon's power.
The monsters have unrivaled supremacy in the sea. That is because the god who once ruled the sea, “Poseidon the sea god” betrayed the gods, and handed control over to the current demon lord. Poseidon was a god revered by monsters of the sea, and human sailors who went out into the sea. She had the power to keep the sea calm, and conversely the power to create storms and drive the sea wild. Sailors worshiped her, and begged her not to cause a storm so that their voyages would be safe. Poseidon the sea god had a heart that was broad like the ocean, and she loved everything that lived in the sea. She was regarded as a carefree woman-god. According to another story, due to the policy of the gods, she was periodically required to create storms to swallow up humans with the sea. The humans had become too numerous, and this was always necessary to make them aware of nature's wrath. However, this didn't sit well with her, because she loved the sailors just as strongly as she loved the creatures of the sea. Perhaps that is the reason why she switched sides and joined the demon lord.
Her magical energy floats everywhere throughout the ocean. Humans who got caught in storms and sunk into the sea had their lives spared by this magical energy. If a woman was going to drown, the magical energy would change her into a "nereid", and she would be welcomed as one of the residents of the sea. If a man was going to drown, the magical energy would protect him and keep him from losing his life. The monsters of the sea would probably come to get him after sensing the presence of a man along with so much of Poseidon's concentrated energy.
Furthermore, Poseidon grants a portion of her power to her priests, the sea bishops. They perform a ritual that reconstructs a human man's body, enabling him to live in the sea. In this way, the men who drown, and the monsters who come to get them become husband and wife, and they are welcomed as residents of the sea. Perhaps because Poseidon loves all the creatures of the sea, the monsters of the sea all love one another. When a new resident, and a new married couple are born, the monsters all rejoice, and celebrate together. Additionally, aside from humans and monsters, there are various other living things in the sea, even ferocious predators like sharks that prey on humans will not prey on humans and monsters who have received Poseidon's blessing probably because they will recognize them as being superior beings.
[...] The deep layer is even further towards the bottom of the sea. It's also called the deep sea. It is even further towards the bottom of a giant hole opened in the sea. No light reaches beyond this point. Also, it's a place beyond the extent of Poseidon's rule. Normal monsters and humans never go near it. According to legend, in the age of the former demon lord, wicked demons of chaos who possessed a power that was even greater than that of the gods existed there at the bottom of the sea. At the time, Poseidon was still one of the divinities ruled by the Chief God, and the rule of the sea was split between her and them. It is said that the demons of chaos were sealed in the deep layer after a war, and even now, the demons of chaos are biding their time to make a come back and once again reclaim dominion over the sea. Those who sunk into the depths of the sea, male and female, are also kin to all monsters, so they've probably assumed the female form, and had their thought patterns changed accordingly, but no one has ever been able to confirm it, and the truth of this is uncertain.
[...] However, presently the sea is ruled by Poseidon, along with the demon lord and the monsters, so sailing is extremely dangerous. [...] Furthermore, just as in the age of the former demon lord, from time to time, Poseidon causes huge storms that rage everywhere making sailing difficult. These storms are not those that kill people. They are caused by Poseidon for the sake of the unmarried monsters. When ships are sunk by these storms, and men are thrown overboard and fall into the sea, as though it were an arranged meeting, there are monsters waiting all around. They are probably taken and brought home by the monsters without exception. [...] The monsters of the sea don't take an interest in men who are overly stained with the scent of a female monster. Because of that, they don't even get caught in the storms caused by Poseidon, and since the wives know the sea even better, they are able to guide. It would be extremely smooth sailing. [...]
Demonic energy can corrupt even a god at times. “Poseidon, God of the Sea” and the “Fallen God” are gods that became monsters, and using their vast power, they have created Demon Realms in other dimensions, changing the environment itself to make an ideal world appropriate for monsters to live in. [...]
A high rank god that ended up betraying the gods and joining the monsters. She was originally a kind-hearted, gentle goddess who deeply loved all who lived in the sea, whether human or monster, but because of the Chief God, she was forced to shoulder the burden of causing storms and tsunamis to thin the numbers of the overpopulated humans. However, she ended up taking the Demon Lord's side because at the same she was being forced to kill her beloved humans with her own hands, the Chief God began persecuting her beloved monsters in the same way.
Sailors and fishermen who live at sea, and nearly all monsters who live in the sea are devotees of this god. Devotees are given blessings by her, and promised safety at sea, and a peaceful life. As of the present, she's become a monster that has demonic energy within her and left the control of the Chief God, so the storms and tsunamis she causes aren't for killing people, they're for hooking up human men with monsters who are looking for a man. It is said that most of the people presently thought to be missing at sea are actually living peacefully in the sea. In the case of men, they've become a sea monster's husband, and in the case of women, they've become monsters themselves. A portion of her power is granted to her faithful followers, the “sea bishops” This power is used in rituals to save humans who've fallen into the sea, and modify their bodies so that they can live in the sea as husbands of sea monsters.