Sunken Journal from Bel, a Mystery of the Shrine of the Coral Tomb.
As I've explored further into 'The Sunken Kingdom' (it's a great name, isn't it? I'm pretty please with myself, not going to lie) I've discovered more about the Sirens. Take this place, for example. I'm pretty sure it's some sort of tomb. The Sirens seem to have taken death, and honouring the dead, very seriously. The pool in the centre contains offerings to those who have passed, and there seem to have been a fair few of them. The sirens were at war with everyone, the Ancients and the merfolk. The rage of the Queen filled all her kind, and they took every opportunity to inflict pain on others. Creating a kingdom from the ruins of anything they could drag down from the world above.