One of the greatest things about Dark Souls 3 is that it takes elements from the games before it and reinvents them in interesting ways. Almost every area and enemy in the game feels new while still managing to be familiar at the same time. At every turn, there is a shot of nostalgia that hides a surprising new twist. One of these new twists is Oceiros, the Consumed King. This strange boss is a reminder that Seath the Scaleless might be gone, but he's not forgotten.
Oceiros was a past king of Lothric that attempted to use his royal blood to achieve great power. In pursuit of that power, Ocerios discovered old research on Seath the Scaleless and was eventually driven mad, trying to emulate Seath's power. His obsession with Seath eventually caused him to transform himself into the grotesque creature seen in Dark Souls 3.
Oceiros Transformed Himself Into a Dragon
Oceiros believed that his royal blood would grant him immense power. His efforts only ended up transforming him into a stunted and sickly version of the creature he was trying to imitate. Like Seath, Oceiros is a pale dragon that lacks the immortal scales of the ancient dragons. Despite becoming a dragon, Oceiros seems incomplete and mutated. He is much smaller than the massive Seath and seems to have bizarre growths across his body.
This seems to be the fate of those that attempt to become dragons. Big Hat Logan, a mage in the first Dark Souls, was also obsessed with becoming like Seath. Logan's all-consuming fascination with Seath eventually drove him mad. It's possible that if Logan had succeeded, he would have turned into the same type of abomination as Oceiros.
Oceiros Performed Disturbing Experiments
One of the remnants of Seath that Oceiros discovered was his crystal magic. Oceiros has replicated this magic. He can cast crystal-based sorceries and release a white breath that summons crystals and inflict Curse build-up on enemies. Mastery over sorcery wasn't the end of Oceiros's experimentation. The king believed that his son, Ocelotte, also possessed powerful royal blood.
It seems that Oceiros has experimented on his own son as well. During the boss fight, Oceiros can be seen clutching something in his hand, while a baby's cry can be heard throughout the fight. Oceiros refers to the infant as the "child of dragons" and repeatedly asks if Ocelotte is hiding from him. It's possible that Oceiros's experimentation changed Ocelotte's body somehow. Shortly after release, fans exploring the found an unused model in the game showing that the child is malformed, as well as audio indicating Oceiros was intended to carry the child during the fight and accidentally crush him at the end of the first phase.
Oceiros's Soul Can Be Turned Into a Sword
Like most bosses in the series, the soul of Oceiros is obtained after defeating him and can be turned into a useful weapon or spell. Oceiros's soul can be transformed into either White Dragon Breath, a sorcery that allows the player to unleash the same crystal breath attack as Seath and Oceiros, or the iconic Moonlight Greatsword.
The Moonlight Greatsword has appeared in every From Software game but Sekiro: Shadows Die Twice and has become an important weapon to the Dark Souls franchise. This weapon does magic damage in addition to the typical physical damage of most weapons. In Dark Souls, the weapon could be obtained by cutting off the tail of Seath the Scaleless. The fact that Ocerios's soul can be turned into this weapon furthers the connection between himself and Seath.