Core Theory is the central theory created by the Slender Nation on their wiki and forum that uses the input from many users to collaboratively create a series of universal properties to Slender Man between Mythos, ARG, and various other canon. This theory is based mostly around the hypothetical existence of the Tulpa Effect, and acts as an in-game reasoning as the the creation of Slender Man in each individual series, describing them as a Tulpa created by the series' protagonists.
The Core Theory is a theory regarding the Slender Man proposed by Robert of White Elephants. The theory focuses largely around the Tulpa Effect and was key in establishing the belief that the Slender Man could, in fact, be defeated, if not outright killed.
The theory goes that, through the Tulpa Effect, we thought the Slender Man into existence. While we cannot simply think him out of existence, we can shift our mental view of him so that he is not invincible.
The best way to do this, Robert proposed, was to use a story as a framing device. Different people would have different roles to fill. Once the roles were filled, the story would start writing itself.
The roles (or Titles) that Robert proposed for the Core Theory are as follows, the descriptions taken from his blog:
Sage: ”There's a dark jungle next to the village. People who go into it never return. The villagers fear the jungle. Then one day a man with a torch goes in, the villagers scream at him to not, but he does anyway. He returns the next night, crawling back, bleeding to death, claw marks on his back. With his last breath he says 'Within the Jungle lives a Tiger, who is twelve feet long, obsidian claws, and has fire in its eyes...but it is just a Tiger and it bleeds.' After that he dies. The villagers no longer fear the jungle after that day. Oh, it's still scary and they take precautions from the huge Tiger...but it's just a Tiger. No longer is it Unknown or Shadows, perhaps one day someone will go and kill the Tiger...but there is no need for pointless fear.”
Mystic: ”The Mystic can equal to -Veteran.- Mystic has fought, stores the knowledge of the past, and continues to give support when possible.”
Hermit: ”The Hermit lives by himself, has developed methods to survive, and is willing to pass on instructions, but has his own priorities, and odds are will not fight as opposed to run.”
Warrior: ”Brave, Bold, Stupid in that headstrong way, Fights for the sake of the fight, Fights without fear, Only fights on This Side, Can push /Construct/ back to Other Side for a while, cannot beat it. A stop gap, and if they become an /Agent/ then there's going to be a lot of trouble.”
Guardian: ”Calm, controlled with knowledge, Fights for others, but will have nothing left on This Side, Fights on Other Side, Stays on Other Side and keep Him there for a much longer time, will return when the Vigil is over, or if a Hero emerges.”
Hero: ”Afraid but pushes on, Foolish but gains wisdom from Sages and Mystics, Fights for those who are important to him, Fights on both This Side and Other Side Only a Hero can kill a monster...but the Hero always dies at the end of the story.”
Bluelight (of Speaking of the Truth) later has begun adding new titles to core theory:
Savant: ”A person of great and varied learning.”
The Filling of the Roles
While not all the roles were filled, Robert did fill a few. These are the roles that were filled:
Sage: Robert (of White Elephants), Jay (of Anomylous Data), and Shaun (of Testing 1, 2, 3) initially filled this role. Robert went a step further with the initial Sage role and said that "one Sage researches weapons, one Sage looks at defensive measures, and one Sage tries to understand the properties of the foe." He referred to the three respectively as the Sword, Shield, and Shroud.
Eventually, Robert passed on these titles to zero (of A hint of serendipity), Maduin (of A Really Bad Joke), and Amelia (of A Road to the Heavens). As of now, only Maduin still seems takes the title to heart.
Mystic: This role was given to Zeke Strahm of Seeking Truth. When Zeke started his new blog, he titled it The Mystic. Zeke is one of the few people who still fully embraces his title.
Hermit: The role of Hermit was given to M of The Tutorial. While many people initially embraced their titles and saw them as an honor, M took offense at being called a hermit. However, others related to the Core Theory still view him as the Hermit.
Guardian: Once Robert passed on the Sage titles, he became the Guardian.
Warrior: While Robert never assigned anyone this role, he has implied that Zeke once filled the role before he became the Mystic. Zero also admitted that he felt this was the role he would've played, if things panned out.
Hero: This is the key role to the Core Theory, and it was never filled.
Savant: Bluelight (of Speaking of the Truth) claimed this title for himself.
Near the end of White Elephants, Robert appeared to be mentally unstable. Eventually, someone (presumably working for the Slender Man) - dubbed "Redlight" by Zero - erased Robert's memories. People looked to the new Sages for guidance, but - due largely to people clamoring to fill roles and claiming to be more powerful than they were, as well as questions of Robert's mental stability - the Sages were, on the whole, dissatisfied with the Core Theory. Eventually, people started drifting away from the Core Theory as presented by Robert.
However, the idea that the Tulpa Effect can create vulnerability in the Slender Man is prevalent among those who know about him to this day.