This book is about the arrival of The Hungering One… signed by 'Umbra'.
After asking Merrick himself, deciphering his drunken journals and speaking to pirates who were there, I believe this is it. This is where they used Merrick's accidental enchanted shanty to summon forth the first Megalodon - The Hungering One! Those pirates avenged poor Merrick's crew, not to mention his legs. But now all of the ancient, deep sharks have awoken…