Actually recommend reading the flashback, it's good---forget about the connection to Ian and read it like a standalone story with a tragic end if it makes it easier---it can be easily read like that.
anyway, if you still want the spoilers
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Brian is a cool guy who worked very hard so that people could have peace
ML is a dragon, who started like an asshole, feel in love with Brian for his aspirations and strong will, and grew to love the land they built and its people as well. He talked with his mom and gave up his inheritance to be able to directly help Brian in human affairs. (Become the king)
Lorkan/Vardinal Brixton (ML's brother) went down to the mortal world to watch the humans as per dragon mom wishes but grew obsessed with Brian for the same reasons as ML, but where ML is impulsive and caring, his brother is cunning and manipulative.
He killed a ton of Brian's people, tried to forcefully break his connection with the ML and kill him afterwards, so he could seek his reincarnation and have him for himself. Brian manages to get away before the connection is broken but he was already dying. ML finds him and he pleads to the ML to forget about revenge and instead protect what they worked so hard to build.
Dragon mom is fucking pissed at Lorkan because he interfered with human affairs, she stripped him of all his power and threw him back into the mortal world as a normal human. The little snake, then, managed to get into contact with the goddess who created the humans and struck a bargain with her to receive her powers while he spread her name (created her church). He, then, finds Brian's reincarnation and you know the rest.
TL;DR: Biggest takeaway of the flashback is that Lorkan is a fucking slippery snake of an manipulative arsehole with no redeeming qualities and wants to cage Ian to himself. Do not believe in his benevolent cardinal facade.
I can attempt, anyone else feel free to correct or add.
Long story short Brian wanted to create a stable country one that prospers and is basically for the people by the people. He at some point recruits Solelite/ Sol /"Jack" to help out and eventually become king of the country via contract that allows Brian access to a lot of Mana (at the cost of his body). They do fall in love with each other after sometime of getting on each other's nerves, understandably so Sol was very uncaring towards Humans back then. It was a lot of trial and error with Brian sacrificing himself a lot for the building of this country and Sol seeing it's affects on him and eventually wanting to help him no matter what. They are quite successful in building the country despite countless of corrupt officials that want Brian dead and to take control of Sol and use him as a puppet due to not only his power but also the awe the villagers hold towards Sol.
Sol ended up losing some powers due to his mom seeing what he was doing and willing to sacrifice for Brian which prompts his brother Lorkan, who we know rn as the Cardinal Heron, to come see why his brother was willing to risk everything for humans. Lorkan/Heron after being a thorn on their side falls for Brian steadily and wanting to keep Brian takes him to a sacred area Kunabullah(?) after backstabbing Brian. Lorkan took the opportunity of Sol being King and tied down to the country to offer escorting and helping Brian set up a third pillar and fulfilling a contract with a neighboring city Brian wanted to connect with for establishing trade. Brian who didn't know Lorkan's intentions (of wanting to severe Brian's contract with Sol) accepted Lorkan's help, despite Sol's hesitance and resistance, due to Lorkan actually helping Brian out before. Lorkan tries to kill everyone on board some ships during the envoy with his powers and takes Brian away, one ship behind finds the remains of the ship and goes back to tell Sol of the shipwreck and not finding Lorkan and Brian.
Last episodes: at Kunabullah Brian finds out Lorkan's intentions and decides he needs to leave before the contract he was with Sol is completely destroyed. The contract with Sol would allow them to kinda ensure they find each other through Brian's reincarnations, so Brian who loves Sol elects to k*ll himself before the contract leaves him. Sol finds Brian at his last breaths and wants to help him survive but it doesn't work out and Brian ends up dying. Lorkan is upset as he wants to enter a contract with Brian to monopolize him and have him, but Brian succeeds in keeping his contract with Sol. Sol and Lorkan's mom (one of the two goddesses who created humans i dont remember their lore that much) upset with Lorkan's interference with the Human world, as he was sent to only observe, takes away everything that makes him an immortal dragon and sends him to "earth" as a human. Lorkan desperately wants to have Brian so he sets a deals with the other goddess to create a country for her on the condition she shares her powers. Implied that she agrees and fast forwards to Lorkan being happy when he finds Brian, now Ian, and can now have him in his "special bird cage".
Sorry for it being so long but here's a tldr in case it's too long
Ian/Brian wants to build a country for people by people because corruption runs deep and is killing the civilians. Sol/ "Jack" is sent to earth to observe and after a while of back and forth Brian enters contract with Sol for access to Sol's powers. They build country after much struggle while also falling in love.
Lorkan/ Cardinal Heron is sent to earth and meets up with Sol and Brian. Sol doesn't like Lorkan and vice versa. After much fighting and Brian being a mediator between the two, Lorkan falls (more like obsession tbh) for Brian after seeing all his efforts for the country and love for Sol.
Lorkan wants contract with Brian so needs to cut off Sol's contract with Brian so takes him to Kunabullah( special place for Dragon immortals) to help break the contract. Brian k*lls himself to persevere contract with Sol (contract ensures they can find each other somehow through Brian's reincarnations). Which leads to now where Heron meets Ian and Sol is "reunited" with Ian.
An aside is that Lorkan and Sols mom is a character and sure does prick you the wrong way sometimes.
I read all of them. Thank you everyone.
Can anyone please summarize the past. I couldn't bear to read all those chapters without ian