this is actually not very far off !!
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Nain is, technically, an immortal being since his body was reborn as the dragon's mate. The difference is that he was firstly born as human. The reason why Amon/the priest want him to get accustomed to killing is because across the centuries, Nain had to grieve the losses of his close people (either because he outlived them, or plainly Amon's fault) and that made him instinctively seek death too, which is a common occurrence with other dragon mates
This is my theory!
Maybe nain was human until he met Amon, but that was 500 years ago.
I was thinking Amon knew Nain for about 500 years (there’s one scene with a lyre and tune Amon heard 500 years ago, and expected nain to know it), and Amon made a deal with the priests, essentially using the priests to keep Na-in by his side (wiping his memories to ease his depression and will to leave Amon)
so the flashback that we see is naik back when he was a human and when amon fell in love w him did he ''mate" w nain so that he was reborn as the dragon's mate and became immortal? and does the agreement amon made with the iyad clans entails that nain will forever remain oblivious to those death etc and lastly what is gwens intention in becoming leshah and is it truly to be close to nain like how does he even know abouthim
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So the thing is, dragon mates are identified as such since birth, and they can attract any dragon in circulation.
When they fell in love, Amon told him that in order to be bound to him as a mate, Nain needed to die and be eaten - Nain was kind of against it because he was still young (and close to his village & family)
Nain argued with Amon, Amon wanted to give him some space and left him. In the meantime, the village was razed to the ground by a nearby cultish village that honoured Amon (they were defying his orders) and Nain was drugged, tortured and offered as a sacrifice. When Amon came back, Nain was irrecoverable, and he died in the dragon's belly iirc. It took him some centuries to be reborn as his mate, thus becoming immortal
Yes, Amon does not want him to remember what happened in the past; the incense thing was initially proposed by the priests, who had discovered its toxicity and potential amnesia effects. Nain is oblivious of the details, but deep down he's suicidal. In exchange, the Iyad lineage has been relevant for ages
Gwen is some 300yo hag who transmigrated from Earth and became a Labyrinth mage (a lot of the recruits are people coming from other worlds); he was researching dragons and specifically dragon mates. Before Nain, he had observed others, but the peculiarity with Nain was that he did not seem hostile to his mate and was actually in a romantic relationship (that's something that rarely happened with others) so he took interest in him. He joined the leshas to be closer to him, it was kind of a love at first sight situation.
Precisely, his research involves obtaining the dragon mates' blood (highly corrosive and poisonous) to be able to create an effective weapon against dragons (weak against their mates' blood)

I think shah nain is a regular human who amon fell in love with and that’s why he’s mentally struggling in the palace. I think the priests are advising Amon to be ruthless so nain can actually become his equal like a ritual to make him a god essentially. Can anyone who had read the novel spoil me? This is what I’m picking up on but idk I’m curious as to why nain is so sensitive