The Conventide Arcanum is The Tome of Magical Secrets. Most if not all of the greatest mages in history used or had at some point accessed the Conventide Arcanum. It possesses a wide variety of information.
Originally given from Vecna to one of his followers. Has been swapping hands repeatedly, for a while associated with Ioun, now no longer specifically associated with any gods. Features in the background of many stories of Great Betrayers.
The most notable time it was used was that one of the Founders of the Silver Legion wielded the Conventide Arcanum in order to seal away the horrible and long lived King Jasminleaf.
Cormin Cherryseeker, Hulo, Vezurz Flare and the owner of Time Not Forgotten found the book about 100 years ago and had it in their possession for about 2 months.
While they had the book they were relentlessly pursued by others looking to take it, and in the end they decided to hide the book away. Hulo at one point tried to destroy it with acid, failing to do so.
They put the book in a lead lined box and dropped it into the deepest part of The Summer Sea (the ocean which houses The Protectorate Nagumat). No one has seen it since.
Cormin Cherryseeker says that The Leyline Map is the last thing he has left from the original Conventide Arcanum, it being one of the pages torn out.