Spoilers to those who did not finish the last clues that led to the Shrine of the Coral Tomb. If you didn't do this last mystery, please do not read any further.
After finding all the symbols, I found the entrance with my crew and heard a voice I never thought I'd hear again since the Pirates Life storyline, after traversing further and solved one last puzzle, the reveal of DeMarco's killer was none other than the Siren Queen!
Her motives was that she wanted to frame the Pirate Lord and make every pirate turn on him for killing his own son.
My thoughts were how the Queen survived after we slayed her and she disintegrated, did she have a way of coming back from death just like Rathbone? Perhaps it has something to do with the red mermaid gem on her forehead, much like the Sirens and Ocean Crawlers who have gems embedded in their bodies.