Soul King

The is the king of Soul Society, whose existence in Soul Society is as symbolic as it is absolute. The king resides in the Soul King Palace that exists in a separate, special dimension in the very core of Soul Society, which is protected by the Royal Guard. The door to this dimension is opened via the Ōken.

The Soul King's sole purpose is to regulate the flow of souls into and out of Soul Society and keep the flow stable. Without the Soul King, the balance is lost and everything that is linked to Soul Society or made in it, as well as the Dangai, Hueco Mundo and the Human World will fall apart. According to a conversation between Kisuke Urahara and Sōsuke Aizen the Soul King is detailed as both a "thing" and "it". Urahara details that without the Soul King's existence, Soul Society would be rent asunder. The Soul King is the "linchpin" and if that linchpin is lost, then the world would simply crumble away and that is the way of the world.

The Soul King is not involved in governing Soul Society, with the king having given the full control of its government to the Central 46 and thus, being uninvolved with the day-to-day affairs of Soul Society.

The Soul King is the father of Yhwach.

Appearance
The Soul King appears to be humanoid in physique, with slicked-back dark hair, oval eyes with black sclera and unusual pupils and very thin eyebrows. He does not have any arms or legs. The Soul King exists within an unusual barrier that is framed on either side by bright white curtains as the focus point of a large, elegant room.

History
At some point in time, the Soul King bestowed the title of "Monk Who Calls the Real Name" upon Ichibē Hyōsube. At some point, he also had restraints placed on his powers by the Shinigami.

The Thousand-Year Blood War arc
Shortly after the Royal Guard travel to the Seireitei and retrieve Ichigo Kurosaki and his Shinigami friends so they can heal and train in the Soul King Palace, the Soul King is approached by a retainer, who watches over the Soul King and inquires if he has awoken when the Soul King opens his eyes. A few days later, Yhwach, his son, arrives at the Soul King Palace and proceeds to slaughter many of the Soul King's vassals and subjects before arriving in the Soul King's room and stabbing him through the chest.

Soon afterward, Ichigo and his friends arrive in the room, where Yhwach reveals the Soul King's situation to them before claiming they are too late to do anything. When Ichigo attempts to help the king by pulling out Yhwach's sword, Blut Vene veins spread over his right side and force him to slash the Soul King across the torso. Yhwach explains Ichigo's actions by revealing that Quincy blood cannot tolerate the Soul King's existence.

As the lower half of the Soul King's body and containment pod fall to the ground, Yhwach explains to Ichigo how he forced Ichigo to attack him by giving the Reiatsu still present in his sword to him, which his blood reacted to. When Orihime attempts to revive the Soul King, her Sōten Kisshun shatters, prompting Yhwach to proclaim that the Soul King will never rise again.

However, in the Seireitei below, Jūshirō Ukitake, who was once saved by Mimihagi, the right hand of the Soul King, initiates Kamikake to save the Soul King's life at the cost of his own. Upon doing so, a large black hand bursts out of his orifices and ascends toward the palace, where it grabs the upper half of the Soul King's body. Shocked by this, Yhwach notes that Mimihagi is the one thing that his eyes cannot predict and demands to know if the Soul King is beginning to feel sentimental toward Soul Society after protecting it for all this time as the eye on the hand of Kamikake begins to close.

Yhwach attempts to destroy both the Soul King and Mimihagi, but Ichigo stops him by grabbing his arm and dispelling his attack, allowing Yoruichi to leap past them and create a Kidō barrier around the Soul King with several seals. Yoruichi notes that Mimihagi has stabilized the king before proclaiming that they will have to replace him. However, when Yoruichi momentarily loses her concentration due to Uryū Ishida shooting her in the shoulder, the barrier shatters, sending the top half of the Soul King flying toward Yhwach, who rips Mimihagi off of it. After absorbing Mimihagi, Yhwach completely absorbs the Soul King.

Powers & Abilities
Soul Regulation: The Soul King's sole purpose is to regulate the flow of souls in and out of Soul Society. Without his existence, all known dimensions connected to Soul Society will begin crumbling into non-existence.

Ōken Bestowment: The Soul King is capable of granting those whom he chooses as members of the Royal Guard with the Ōken, a special tool that allows one to enter the dimension of the Soul King Palace whenever they wish. The king achieves this by literally turning the bones of those he selects as Royal Guard members into this Ōken.

Precognition: Like his son, Yhwach, the Soul King has the ability to see into the future. Yhwach theorizes that the Soul King has seen all the events of the Quincy Blood War, up to and including him invading the Soul King Palace.