Actually the book didn’t mention Nyriad, Maya just said it in her sleep. Ava overheard her while snooping. Maya didn’t know she had even said it, or who Nyriad was.
The Eridian Writings dealing with the Sirens specifically never note the number until the Seventh is mentioned:
Nyriad
“[Nyriad] You. You, who hears me, eons after my time. I have set these words down, even as my mind fractures under the weight of what I have done. Find my words, and remember the their truths in my place. For I am the one who put the Destroyer in its chains. I am the who murdered the Eridians. I am Nyriad.”
Brighter
“[Nyriad] When my atrocity was complete and I returned home, I found no family to rejoin. The others had fled. Our braid.”
Purpose
“[Nyriad] As the Siren sisterhood, we were the keepers of many answers, but our own nature remained elusive. We never knew our origins, nor our purpose. In this way, we are still undeniably mortal.”
Braid
“[Nyriad] I have never known a time without my sisters. Even when one passes and a new Siren receives her markings, she always finds her way to the sisterhood. It seemed the will of the universe that Sirens be together, our lives entwined like an unseen braid.”
Witch
“[Nyriad] I am Siren. Like the Destroyer, my kind has been called by many names. Oracle. Angel. Witch. Rarely are the names used kindly. I used to hate the ones who feared us, believing them ignorant. I think now, after what I have done, I better understand.”
Hunted
“[Nyriad] Sirens have been hunted for as long as they have existed. Hunted out of fear, out of jealousy, out of misguided greed. The sisterhood was our way of passing down the craft of staying hidden, and staying alive when hiding was no longer an option.”
Knotted
“[Nyriad] No thread is ever truly cut. It can be tied to another, whether knotted by careful fingers or tangled by chance. Every Siren must decide what will happen when their thread reaches its end.”
Burden
“[Nyriad] To inherit a Siren’s gift is also to inherit her curse. This is the choice of a Siren present with death–pass on the gift and burden someone worthy, or release it, and burden an unknown soul who has no idea that the entire universe is about to want them dead.”
Secrets
“[Nyriad] With Pandora sealed, and the Destroyer chained within, the greatest of the Eridians’ secrets is put to rest. It would be folly for me to think, though, that this is the last of them. There are countless other wonders or terrors they hid across the stars before my time. For every Graveward, Traveller, or Timekeeper, there are surely a dozen more slumbering in forgotten Vaults.”
Warning
“[Nyriad] You. You know now all that I know. This was an epitaph, not for myself, but for the world I knew. I have brought ruin to the Sirens, murdered the Eridians, and imprisoned a primordial force of the universe. Perhaps I have earned your hatred or your sympathy. But that is not why I write these words. I have told you the truth, no matter its hard edges. Before I can put my fractured mind to rest, I must pass one more warning. [Nyriad] You must never find the Seventh.”
FromDarkHell’s site doesn’t have the key writing for me, Brighter