The Chronicle is a large book created and owned by the Chain of Acheron. It contains a record of the members and adventures of the Chain, and is very important to the group. If the Chronicle is lost or destroyed, the Chain is no more.
The Chronicle keeps track of the progress of the Chain throughout its entire history, most of which is not written in Common.
The Chronicle can be used to help the reader make knowledge checks.
If the Staff Sorcerer of the Chain studies the Chronicle for eight hours (i.e. a long rest), they can choose a spell they know that can be cast at higher levels, and cast that spell at one higher level without using a higher level spell slot.
Several entries of the Chronicle can be found on MCDM's website, acting almost like a blurb for the episode the entry is representing.