Intellectus refers to a mode of existence which allows instant knowledge of anything by merely thinking about it. Having Intellectus is a feature of a small number of powerful supernatural beings.
Intellectus allows an immortal being to gain access to any knowledge it needs. It is a mode of existence that allows a few rare beings to view reality as one piece, place and moment. They no longer search for knowledge, they just simply know, seeing the whole picture.
There are several limitations. For one, the being must know what they are looking for, and it takes many millions of years to acquire, and beings only have Intellectus on specific subjects; such as Demonreach on its island and the Naagloshii on torture.
In the seriesEdit
The island Demonreach has Intellectus limited to its own shoreline. Harry Dresden receives a limited second-hand form of it from the island's genius loci, allowing him to know anything on the island, as long as he thinks of it.
According to Word of Jim the Skinwalker that Dresden faced in Turn Coat has a limited form of Intellectus that focuses on how best to torture its victim. In the novel, it abducts Thomas Raith because it knows that hurting him would be an effective way of hurting Dresden, and that destroying the humanity of Thomas would be a good long term method of hurting him too.