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.

It's unclear what the Soul King actually does, other than that the king has a specific system of "specialty duties" that are actively carried out. These duties apparently require a very high level of competency or skill of some sort. The Soul King and the Royal Family are apparently completely indifferent to 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.

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 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. 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.

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 loyal retainer. The vassal 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 vassal 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 wounded Soul King 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 chest as Yhwach reveals that Quincy blood cannot tolerate the Soul King's existence.

Powers & Abilities
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.