The Metareality Core is an indeterminately sized core containing all of the infinite information regarding the Metareality-Web.
On the inside of the core is an infinite library containing all information of the web its contained in. It's destruction would cause an implosion of all of the Metareality-web and The Eraser, The Pencil and The Paper would promptly explode, which would likely open a hole in The Table causing The Shadow to be free.
The Metareality core's energy causes matter around it to chaotically swirl around it, this is because of its huge concentration of Primordium Delta, approximately 92%! The rest of it's composition is of small Information Bubbles containing information about laws of physics, matter, and other necessary details. Again, much like a library, these bubbles are much like books and are organised in sections.
The Paper is in fact directly linked to the core. Any information the 'realitators' within the paper get, will be directly sent to the Metareality Core to then be sent to be changed to the Metareality-Web.