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 then train in the Royal Palace, the Soul is approached by a loyal retainer. The vassal watches over the Soul King and enquires if he has awoken when the Soul King opens his eyes. A few days later, Yhwach, his son, arrives at the Soul King Palace, having defeated the Royal Guard and proceeds to slaughter many of the Soul King's vassal and subjects. He arrives in the Soul King's room and plunges his sword through the barrier and the Soul King's chest. The Soul King remains motionless and unresponsive despite this injury and Yhwach's mocking taunts.

Yhwach is soon interrupted by the arrival of Ichigo and his friends, who have been in pursuit of him since he left for the Soul King Palace. He mocks the teenagers, insisting that Ichigo is too late to change anything as he asserts that the Soul King is already dead. Ichigo rushes to Yhwach's blade as the Soul King's son goads Ichigo into removing the blade and thus bring destruction to the Soul Society with his own hands. Ichigo continues to remove the blade from the Soul King's chest but the Quincy blood within Ichigo activates and takes control of his movements, forcing Ichigo to slash through the Soul King, despite trying to save him. Yhwach stresses that Ichigo's Quincy bloods rejects the existence of the Soul King so they should both now watch as ruin is brought to the Soul Society, which starts to quake and crack.

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.