
Let me see if I can summarise it. There is another reader in here that can summarise it better. You will find them trough the comment section since they are in here for every chapter (let me check their name.I will write it on another comment).
But back to the explanation.
On the first timeline Sion was in love with Parvellone. And Sion had the Sain Zion stigmata. It's like a tattoo or something that proves that he is the chosen one from God. And Parvellone is the angel of destruction. For the holder of the saint Zion stigmata to be awaken, The angel of destruction must die, aka Parvellone (wich happens on the first timeline, Parvellone died.Parvellone changed Sion memories from the first timeline, so what he remembers happening back then isn't what really happened).
About Sienna.
The Mayee house wanted control over the empire or whatever, and having a holy lord from the house Maye was a good method to achieve that. So they put Sienna on the throne.
However, Sienna's didn't have a stigmata of his own
. So the Maye house took children and tried to transfer their stigmata to Sienna's body. When Sienna discovered about Sion stigmata, he kidnapped him and tried to transfer his stigma to him(peel the skin and put it on Sienna's or smt like that).
Back to the first timeline, Sienna wasn't able to transfer Sion stigmata, Parvellone found Sion, I don't remember what happened there really, but they both die, activating Saint Zion stigmata. Sion goes back to the past, but on Sienna's body.
Sion original body disappears.
Sienna's soul no longer inhabitants that body.
Also, Sain Zion stigmata holder was destined to become the holy king, but Parvellone interfered with that on the first timeline, trying to change Sion destiny.
This is literally God saying, well sorry it can't really be changed, and rewinding things.

Spoiler
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Sorta! He'll find out overtime when he digs through the history of their kingdom and when he finally finds the contract between parvel and the prev holy king before siennas. Via that and history books, Sion sorta piece things together. He also finds out about Parvel's grand plan and finds a way to basically prevent it from happening.

I'd recommend saving chapters for now. What i'm doing tbh but for diff reasons since i've already the novel. Manwha seems to be pretty far from the climax of the story.
The story is kinda like a mystery genre?? Since Sion overtime has to figure shit out along the way and the mystery of Parvel and the truth is one of the biggest ones to be revealed later on.
I’m not gonna lie i’m a littttle confused with the whole Parvellone situation and how he was involved with Sion and how it relates to him now in Sienne’s body…