Decoded!

Cassi escorted the heroes to Vocar's cell. Not a prison cell, but more like a monk's cell. He was barely clothed and the stench coming from the corner was almost more than they could bear.

Vocar was not forthcoming about anything, particularly to start. I enjoyed playing him, I hope the players enjoyed my RPing. In the end, the heroes did persuade him to read/decode the book for them.

As they assumed, it was written in a secret code readable by the servants of Vecna. In fact, to read it, Vocar closed his good eye, lifted his eye patch, and read it using the empty socket. The empty socket idea occurred to me able 12 seconds before I said it. I was very happy to have thought of it.

In the end, the heroes learned:
Damakos' attack was designed to be a distraction to them.
Osterneth is leading a massive astral vessel in an attack against the Raven Queen. The plan is for her to distract the forces of the queen and allow Vecna the chance to slip in and defeat her.
It is likely the current Exarch of Vecna, Falazure, an ancient, black, draco-lich will also be on or near the ship.
The heroes have 7 days to plan their assault on the ship.