The Soul King (霊王, Reiō) 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 appears to be humanoid in physique, with slicked-back dark hair, oval eyes with black sclera and unusual pupils and very thin eyebrows.
The Thousand-Year Blood War arc
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.