Sunken Journal from Umbra, a Mystery of the Shrine of Ancient Tears.
It took some time, a lot of persuasion and following threads to loose ends but I finally got the answers I was seeking. Since I last wrote, I have discovered this new type of merfolk called Sirens and that they have a dark and complicated past. The chest that Dimitri found contains the soul of their King, which must mean the Silver Blade was dragged below. I can only hope some of the crew survived. The sirens have been sleeping, for how long I cannot tell, but something has awoken them — and their Queen still rages for her lost love, who is forever just out of her reach within this chest.