Poseidon

Poseidon is the God of the Seas and Earthquakes in Greek mythology, and brother of Zeus, King of the Gods, and Hades, God of the Underworld. A son of the leaders of the Titans, Cronus and Rhea, like his other siblings he was swallowed by his father, due to Cronus's fear that his children would overthrow him just as he had overthrown his father Uranus. After the youngest child Zeus managed to escaped this fate and had grown up, he caused Cronus to regurgitate his siblings, including Poseidon, and subsequently led them in a war against the Titans. After the Titans were defeated, Zeus, Poseidon and Hades drew lots to decide their domains, with Poseidon receiving control over the seas, Zeus the sky and Hades the underworld, with the land being shared between them.

As with the other Olympians, Poseidon plays a role in many stories of Greek mythology and their derivatives. One of his sons was the hunter Orion, who gained the love of Artemis and as Poseidon's son was able to walk on water. Poseidon gave the divine horses Balius and Xanthus to Peleus as a wedding gift, who would later be used to pull the chariot of Peleus's son, Achilles. Minos, king of Crete, had a promise with Poseidon to sacrifice a bull presented from the sea god, however one year he broke the promise and sacrificed another bull instead. For breaking the promise, Poseidon cursed Minos's wife Pasiphae, causing her to have sexual desires towards bulls and resulting in the birth of Asterios.

In the story of Medusa, which differs from her actual life, Poseidon's pursuit and violation of her is responsible for her subsequent curse and transformation into a monster by a goddess, either Poseidon's wife or Athena, and his children with her, Pegasus and Chrysaor, are born from her neck after she is decapitated by Perseus. In another story, he had his way with the Lapith Caenis but granted her a wish as compensation, transforming her into the male Caeneus and giving him impenetrable skin, so as not to suffer the same fate again.

In Fate/Grand Order Cosmos in the Lostbelt:

Formally, the Planetary Environment Modification Plant-Ship Poseidon, he was in charge of the modification and control of the oceanic environments. Within the world of the Lostbelt, there was a shift from the Greater History of Man in 12,000 BC that ultimately led to the Age of Gods continuing into the modern day. Having survived the Olympiamachia, Zeus overwrote his cores as punishment, forcing him to serve as one of the Atlantis' three defense layers, being the gatekeeper of the Big Hole that leads to Olympus.