I am sensing a bit of... anxiety of fear in the SJM fandom, over the released snippet from acosf.
Let's try and diffuse this.
This has been known as the Nessian book from before the title, cover and synopsis were released. This has probably been known as the Nessian book since before SJM had a plot.
There has been a lead up to them being together since long before the acotar series ended.
SJM frequently pairs her protagonists with guys who initially have conflict, but ultimately inevitably fall in love
See: Feysand, Rowaelin, Manon/Dorian, Lysandra/Aedion, Lorcan/Elide, Chaol/Yrene, and unfortunately *sigh*... Hunt and Bryce (I don't like them together, sorry not sorry).
And in all of these, the protagonists initially are attracted to another character, while also admiring the love interest. There is nearly always a dummy love interest ^ (see aforementioned attractive character). While the protagonist sorts out their feelings and their own healing from trauma, with the help of the actual love interest, they become friends. Now, this is an important step. They are ALWAYS friends just before they fall in love. ALWAYS. No matter how bad the tensions are between the protagonist and love interest at the beginning, and they have been BAD, they always work to become friends first.
Idk if Cassian and Nesta are mates. Idk if they know. Neither do other characters, but they all have their suspicions. But we have to remember, both Nesta and Cassian are incredibly private, vulnerable and secretive people. This could perhaps be the symbolism of the cover - with masks. Both use coping mechanisms to diffuse tension, deflect embarrassment and ignore trauma. Nesta lashes out with anger, Cassian with humour and arrogance. This is important. They're both TERRIBLE at communication. So they both need to work on that, and open up to each other about their anxieties, trauma and fears. This is linked to the becoming friends bit. But, that will obviously take time. Which is where the Eris scene comes into play. It is well known that Eris assaulted and abused Mor, and that the entire Night Court, including Cassian, are FURIOUS about this. It is also well known that Cassian, Mor and Azriel have had a 3 way flirtation/hookup system.
And because Nesta has been isolating herself from the Inner Circle, she might not know this. I don't know, I can't remember. Sue me. 🤷♀️
So, she and Cassian have an argument based on lack of communication and misunderstandings, and while at a masquerade ball (idk, context 🤷♀️), she knows of Cassian's aversion to Eris and decides to get back at him.
Perhaps because she's jealous of Mor or something and knows that Eris can be linked to that messy pile of shit.
This ^^ is the official synopsis of acosf on GoodReads which 100% sets up Nessian.
Now, let's look back at acomaf, for comparison purposes. Not until halfway through do Feyre and Rhys get together, after becoming friends. For at least the first third of the book, she and T*mlin are together. Although both traumatised, and on Feyre's part, currently being abused, both she and T*mlin want to be together, until a certain point. They even have sex. Despite all of this, it is not a healthy relationship, and does not last, nor should it have. The purpose of looking at this, is to compare it to the tiny snippet of a scene from acosf. T*mlin and Feyre have sex. And when reading that scene out of context, it could be seen as romantic and meaningful. However, from the context of this book, we know that it is not a healthy relationship. It is incredibly toxic and is built ENTIRELY on control. But without the context of the entire book, you may not know that. We only have a tiny snippet of one scene from acosf. We have no idea what is happening in that scene between Eris and Nesta.
Therefore, we cannot say that there is anything between Nesta and Eris. In fact, we can say that considering Nesta's past trauma, and Eris' abusive behaviour, we can be sure that if Nesta knew what Eris had done to Mor, she would cut of his head and drink his blood from his skull with a smirk on her face.
Almost finally, the scene is told from Cassian's perspective, who looks at it with a mix of devotion for Nesta, and anger and jealousy for Eris. Considering Cassian's admiration for Nesta, it makes perfect sense that he would describe her this way ^^ no matter who she's with. "Glowed as if she had devoured the moon too."
And finally, we're only seeing the end of this scene. We have no idea how they came to dance together, or why. There must be a reason SJM didn't give us the beginning, like major spoilers. And cos obviously the end has enough to terrify us.