The Quill of Correspondence is a favored magic item of writers, scribes, barristers, spies, and assassins. A weird mix to be sure, but it is due to the quill’s ability to transmit messages secretly over potentially long distances that makes it useful to all of the above.
The Quill of Correspondence itself will radiate magic, but is unremarkable in and of itself at first. It will radiate a minor aura of Conjuration and Prescience magic if examined by someone capable of reading such auras, however, it operates only as a normal quill unless the following steps are taken. A command word or sigil, chosen by the maker of the quill when it is created, must be written onto two separate surfaces. Many tend to prefer using a book or tome, though any surface that will allow writing upon it will do. After the sigil or command word is written upon the surfaces then anything written below the sigil (or command word) by the quill on one surface will also appear written upon the other.
Now here is where often different Quill of Correspondence may function in different manners. You see most Quills of Correspondence will have a second command word that when spoken (with the Quill in hand) will remove the sigil/command word immediately from both surfaces where it was initially written, and this severs the magical connection the two surfaces had. That said, some Quills of Correspondence are also made in such a way that any text transcribed by them will also disappear when this command word is spoken. Not all Quills have this feature, though.