Books of Summoning
When the inhabitants of the Outer Planes started to wreak havoc on the Material Plane, Celle decided it had been enough. Instead of wiping them all out he made an agreement. A collection of powerful books were born into existence. For a majority of the denizens of the Outer Planes, they can only appear in the Material Plane if they have been summoned by one of four books. Every entity that grows in power enough to compare to a majority of their peers, they will manifest a book of their own. There are books that can summon one of the Demon Lords or Devil Princes themselves. Because he chose not to destroy them he restricted their access to the Material Plane. The books were his way of coming to an agreement. As long as the Outer Plane being is alive, their book will remain in the Material Plane. If the book is destroyed, it reappears somewhere else on the Material Plane after the individual who destroyed it dies. Destroying one of these books is an incredibly difficult feat as most damage doesn't work on them. Their magic is held in place by Celle's own spells. It makes it impossible to decipher and copy. When a magic-user wants to summon one of these beings they use the book in a ritual that is outlined by the manual. It will first have to be deciphered from the language of the people it summons. For instance, you must translate the instructions from a Book of Demon Summoning, you would need to know Demonic in order to understand them.
On the continent of Itherion, Elmoth, Demon Lord of Illusions was summoned to the Material Plane. That summoning gave rise to Elmoth as the Demon King and a 400-year reign of demons on Itherion. Most don't know of the role of the Summoning Books to this event, it is a very well kept secret by the Libraries of Celle. It has been confirmed that several Summoning books are in the possession of the Libraries of Celle.
These books are seen as dangerous items. While destroying them could delay when the being is summoned, locking them away has a higher chance of keeping it out of the wrong hands.
Until the destruction of creation.