

I have a theory/prediction about Elain for the upcoming books! (contains ACOWAR, ACOSF, & Az BC spoilers)
Soooo I wrote the entire behemoth to post to r/ACOTAR but apparently I don't have enough karma points there yet since I'm still new. SO, I'm gonna share it with you all because I'm hoping you'll see the vision!
I'm almost done with ACOSF (on chapter 75, first time read-through). There is a conversation exchange in chapter 62 has got my brain BREWING.
Please walk with me… let’s take a bit of a journey.....
The Sister Mountains - A Trial For Each Sister
ACOSF - In chapter 62, Eris says to Nest and Cassian, "There are three of them, you know. Sister peaks. This one [the sacred mountain], the mountain called the Prison, and the one the Illyrian brutes call Amriel. All bald, barren mountains at odds with those around them." He continues, "We don't know why they exist, but do you not find it strange that two of the three have underground palaces carved into them?...Unsuprisingly, the Illyrians were never curious enough to see what secrets lie beneath Ramiel. If it, too, was carved up like the others by ancient hands."
This has got my wheels spinning, of course. Why three mountains? Why are they called the Three Sisters? The presence of the number three seems to be a reoccurring theme in ACOTAR. Three brothers, three sisters, three mountains, three stars.
So far, two out of the three sisters have completed a trial/test that culminated in a fight against a villain, resulting in that villain's death within or on one of the Sister Mountains. For Feyre, it was Amarantha and her trials inside the Sacred Mountain. For Nesta, it was the Blood Rite and facing off with Briallyn on Amriel.
That only leaves Elain, leaving open the possibility that she has her own trial and conflict that will take place on or in a Sister mountain.
The third mountain not yet 'conquered' is the Prison, but that is a place that has been accessed/utilized by Feyre and Nesta as well. They both went in to retrieve/gain something powerful that would aid in an upcoming battle - Feyre recruited the Bone Carver, and Nesta retrieved the Harp. They also both received visions while in the Prison - Feyre saw a vision of her future son, and Nesta saw the presence of a fourth Dread Trove. The Prison seems to be a place to gain/receive things of importance; so I am not entirely sure how it might contain a trial for Elain.
The other possibility, of course, is the hidden ruins beneath Amriel. Eris's little history lesson was a giant flag for something of future importance. There is still a mountain left unexplored, just how there is a sister who's story has yet to be told.
Either way, I'm suspecting that Elain will likely have some sort of trial/test that she must endure, either having to do with her powers as a Seer, OR something she must find.
Finding an Object of Importance - The 4th Dread Trove, or Koschei's Heart?
At first I'm thinking maybe it'll be the fourth dread trove that Nesta saw in her vision; but THEN I remembered something that Elain had said in ACOWAR during her mind-muddled talk with Lucien:
She looked away - toward the windows. "I can hear your heart," she said quietly.
He wasn't sure how to respond, so he said nothing, and drained his tea, even as it burned his mouth.
"When I sleep," she murmured, "I can hear your heart beating through the stone." She angled her head, as if the city view held some answer. "Can you hear me?"
He wasn't even sure if she truly meant to address him, but he said, "No, lady. I cannot."
I know that in the mythology, Koschei hid away his heart like a horcux in order to be unkillable. So is it Koschei’s heart she’s hearing? The bolded words definitely make this exchange stick out for me. I truly wonder which direction the window was facing as she was looking out of it. Did the window face towards Amriel/the Prison? She asks, “can you hear me?” Not “can you hear my heart?” As if she is speaking to someone else, calling out to them and seeing if they answer, if they know she’s there.
We know so little of her powers, there's no telling for sure what she even meant by "hearing a heart." I don't think it's a mating bond thing... I went back and looked at Rhysand's death in ACOWAR, and Feyre described it as such:
It wasn't there.
That thing that rose and fell with each breath. That echoed each heartbeat.
The mating bond.
It seems as though the mating bond, while tied to his heartbeat and breathing, doesn't allow Feyre to hear his heartbeat directly. It seems more like a resonance, a vibration passed through the bond, which if I recall correctly, is often described as a thread. And when she was begging for Tamlin to help...
I laid my head on Rhysand's chest, listening for any kind of hearbeat through the armor.
And when she talked to Rhys, begging him to come back, her head still on his chest...
And in return... a thud sounded.
I opened my eyes. Another thud.
And then his chest rose, lifting my head with it.
The only time she's able to literally HEAR his heartbeat is when she physically has her ear to his chest.
So for Elain to hear a heartbeat beating through stone? Maybe the "forgotten melody" of book 7 is Koschei's heart, which has been hidden away for so long that there is no one left alive who could remember the sound of it. Finding it will be the key to defeating him in book 8. This means that his heart can either be in the hidden, forgotten ruins beneath Amriel, or in the Prison. The Prison is still a possibility; Nesta had told Cass that Fae had been trapped literally within the stone. Also, Eris had said the Prison had a palace carved into it, and Cassian had responded, “I’d hardly call the prison a palace. Just ask the inmates.” Eris’s response? A mocking smile. It’s giving “you really don’t know, do you?” Vibes. (Eris, you sneaky, mysterious bastard…) Maybe it’s possible that there is more beneath the prison. After all, the harp was hiding behind a fake wall for ages. And finding Koschei’s heart in the Prison, right under everyone’s noses where they've been multiple times, has a sort of “full circle” poetry to it.
Elain's Trial
As for Elain's trial/test, I'm suspecting it would likely have something to do with her Seer powers, since we've seen and learned very little about that so far. Feyre's trial with Amarantha required her to use her wits and quick thinking - beat a wyrm, solve a riddle, stab Tamlin's stone heart, etc. Nesta's trial had to do with combat and becoming a warrior. Both Feyre and Nesta's trials were vital steps towards them stepping into their own powers and strengths. This will be the same for Elain, which is why I think it will have to do with her magic and how she's able to utilize it to establish her own agency/independence. And I'm 99% certain that Elain's trial is going to play some important part in finding Koschei's heart in book 8.
Either way, Elain is going to go through a trial and it'll take place within a Sister mountain.
Elain's Fight
Elain will also have some sort of fight/conflict. This won't be a warrior's battle like Nesta, or a battle of wits like Feyre. I think that Elain's conflict will be centered around intelligence and strategy and it’ll be magic-based, with a focus on stealth, Seeing, and maybe even a mind-magic element..? This has already been laid out with the groundwork of establishing her ability to see the future and her close connection with Azriel. For example, when she kills the king of Hybern with Azriel's Truth-Teller, she comes out of a shadow. And in ACOSF, Elain startles Nesta by how quietly she appears behind her when Nesta arrives for Winter Solstice. She's able to observe and read people exceptionally well, as evidenced by how she described the story of Nesta seducing the Duke, and during her talk with Lucien she said, "no one sees me. Not really." (or something to that effect), I.e., she often goes unnoticed and is generally underestimated - good elements for a spy, which she can learn to use to her advantage in book 7.
Elain's Opponent - Briallyn's Spy/Spies
Now, with the presence of Briallyn's spies which was left unresolved in ACOSF, it sets us up with a like-minded opponent for Elain. We know that Briallyn's spies had infiltrated so well that they were able to identify Nesta's friends, Gwyn and Emerie. Considering that Gwyn never left the House of Wind before Nesta's chat with Briallyn while retrieving the harp, I think it's safe to say that the spy/spies are priestesses that have infiltrated the library beneath the House of Wind. This would be an incredibly impressive infiltration and would give Koschei access to a wealth of knowledge that he would need in order to free himself from the lake.
I'm putting my money on Merrill and maybe even Gwyn, although I'm suspecting Gwyn is a good person being forced into a bad situation (remember, she has a sister that supposedly died - but we also know that Koschei traps girls into swans at his lake... I need to back and double check Gwyn's story to confirm if there's a possibility that it's been left intentionally vague or not). ACOSF has reoccurring themes of wind and song/music, including Azriel’s bonus chapter. Merrill's magic is tied to the wind, which Koschei supposedly sends his messages on, per Vassa. Gwyn clearly seems to have some sort of song-power, evidenced by Nesta receiving the vision of the harp during Gwyn's singing, as well as how Azriel's shadows react to her in the BC. Merrill and Gwyn work closely together, Gwyn doing Merrill's bidding while Merrill is fervently researching for information on the Valkyries... I wonder if there's something else she's searching for too?
Overall, wind and song, forgotten knowledge and spies... it all connects and aligns with Book 7's title, A Court of Forgotten Melody.
I'm thinking Book 7 will culminate around Elain going to a Sister mountain (Amriel or the Prison), likely with help (Nuala and Cerridwen, Azriel, or maybe even the Valkyries - who are priestesses, btw) in order to master her magic through a trial of some sort which will somehow tie into finding Koschei's heart. This trial will culminate in a conflict with the main spy, likely Merrill, maybe Gwyn, although I personally love the idea of Gwyn joining Elain as a Valkyrie for protection but is secretly a spy, and ends up turning on Merrill, heh heh.
Anyway, either way, I'm having fun with this! I know these books have been out for a while, so I'm pretty sure people have already considered these things, but I'd love to hear everyone's thoughts!