The Tall One is a vital part of the Aspect Theory . It is the combined being of the Slender Man, the Guardian, and several other entities. It has never been seen, and it is unknown whether the said entities actually look like the Tall One.
Not much is known of the Tall One, other than it has been in this universe as long as mankind. Choices claims to know more about this being, but refuses to say anything else other than the Guardian has prevented him from speaking.
The Tall One exists in many different realities as different entities, or aspects, with the Slender Man as the most powerful of them all. The current reality of Corrupt Secrets contains the Guardian aspect.
All of the aspects contain a primal desire to combine and form the full being of the Tall One, but each wishes to have dominance as well.
If the Light Theory is true as well as this, all also wish to find a strange being/object known as the Light.