The Leviathan is an ancient deity that exists outside the mortal plane. It seeks to enter the human world, which is separated from him by an all-enclosing barrier protecting it. However, the Leviathan is not just another typical demon or eldritch god bent on world domination. It is formed as the manifestation of the tension between the modern world and the premodern world and the rage it creates. Originally it existing as the concept of there being something greater than man, such as gods or the supernatural, that could not be controlled or reasoned with. Modern ideas spurred on by the scientific revolution and the enlightenment encouraged humanity to understand their surrounding environment with a new perspective that focused on rationality and proven facts based on trial and error analysis. The new relationship between humanity and the world began to displace older ideas like religion and superstition. This has led to a time of increasing religious indifference, where people are more concerned with their number of likes on social media accounts and only token tribute is paid to old practices of worship. This concept of the Leviathan was then shaped by the encroachment of modernity into the old world and the clash between the two. The Scarlet King solidified into a sentient entity after it was defined as such by the SCP Foundation, a group devoted to securing anomalies in the interest of protecting current civilization and sustaining its normalcy.
An organization known as "The cult of the Leviathan" seeks to reincarnate their deity on the mortal plane directly in the form of a human child with Leviathan's soul. The group seek to return the world to its premodern age where humanity feared the unknown and looked to gods and deities for their sustenance and continued survival. While the cult is powerful and has access to resources, it is small and operates in secret to avoid persecution by governmental forces who see them as dangerous madmen. After being born into the world through mortal means, he will grow up to become the living manifestation of their god and subjugate humanity to his rule, with the cult at the helm of power. As the Leviathan became real, so too did a history of its existence reaching back to the Dawn of Creation, altering history to include it. The cult has essentially created a reality warper who can retroactively rewrite the world's timeline, inserting itself into various points of our history's past. As a result, several cultures and empires have risen and fallen in the revised history, displacing important events with new ones and essentially being the narrator of its own story.
As this being has the ability to control the timeline, it stands to reason that it can manipulate things to make itself the winner of past events, establishing the cult as a far more prominent group in the modern day instead of a small organization forced to operate in secret. It could even make itself an already summoned entity who has already achieved world domination and humanity's enslavement. What could prevent a deity who can alter time from making itself the winner?