Explanation
Details of Incident in Chapter 1
The true culprit behind this incident was Drago Walsroder, king of dragons.
Around 0:00, Drago, intending to stop Eida, left the parlor to go up to the second floor and head towards the throne room. Before Drago entered the throne room, Lily was already there. Determined to find the magic book, she sneaked into Eida's bedroom. Val is catching up to Drago and climbing the stairs to the second floor. Marine was casting her magic and transformed into Eida in the dining hall. Meanwhile, Stan is in Eida's study room.
Around 0:10, Drago fulfilled his promise by killing Eida in the throne room. At the same time, Val spotted Eida, who is actually Marine in disguise using her Water Mirror magic, and dragged her to the dining hall.
Around 0:20, Drago dug his claws to Eida's back, targeting his vital points. His heart fatally wounded, Eida fell facing the floor and breathed his last. During this time, Stan was standing outside the throne room, waiting for a chance to talk to Eida again.
Meanwhile in the dining hall, Val readies his claw to kill Eida (Marine) who showed no intent of listening to him. Coincidentally, he dug his claw towards Eida (Marine)'s heart from behind, just like Drago. (Eida) Marine ran away, but Val didn't chase him and stayed in the dining hall.
Around 0:30, Drago searched for a way out of the throne room, and found a passageway below the throne. He goes down to the basement through the passageway and heads towards the parlor from the study room. There, he heard Lily's magic activating.
Meanwhile Stan, who was still waiting in front of the throne room, shocked to see Eida (Marine) ran to the treasury with a severe wound on his back. Curious on who has done it, he peeked inside the dining hall and saw Val with his claws covered in blood.
Upon realizing that Val had noticed him, Stan ran away towards the throne room. Inside, he found Eida's corpse lying on the floor, with the same wound he saw as before. Stan thought that Val or the other race representatives had killed him to stop him going further becoming an evil king, and so he decided to fake the crime scene by leaving the Royal Dagger in Eida's pool of blood.
At this time, Lily who was searching for the book in the archive room goes to Eida's bedroom and peeked through the throne room from the keyhole and saw Eida's corpse. Unable to withstand the sight of blood, she accidentally activated her magic Memory Wipe.
Around 0:40, Drago headed towards the throne room again after losing some of his memories. Marine too began heading towards the throne room from the treasury. Hearing footsteps, Stan quickly fled to the bedroom and get out from the archive room.
Around 0:50, upon finding Eida's corpse, Marine let out a scream. Drago heard it and entered the throne room. Stan tried to naturally enter the throne room, but Lily saw him sneaking out of the archive room.
Around 1:00, Val, Lily, and Stan all gathered in the throne room.
And discussion to find the truth started.
It is assumed that players can safely reach the truth by gathering info and organizing the timeline of each character.
There's two important points that can help them reach the truth easier is; 1. Find out that Marine was transforming into Eida, and 2. Realizing that the study room in the basement connects to the throne room.
Hints from Evidence
Evidence No.1 "Stan's Memory" was his memory about the medicine Memory Mend.
Evidence No.2 "Traces of Magic", the magic in question was Lily's Memory Wipe.
Evidence No.3 "The Royal Dagger" was Stan's attempt in faking the crime scene.
Evidence No. 4 "Bloodtrails", the blood is of Marine's after she was stabbed by Val.
Evidence No.5 "Scale" is Dragos' when he was washing away the blood on his claws in the well.
Evidence No.6 "A Strange Magic Book" was a how-to book about controlling one's magic.
Evidence No.7 "Traces of Magic" was traces of Marine's Water Mirror magic.
Evidence No.8 "Empty Container" was a water bottle that Marine always brought with her.
Evidence No.9 "The Throne" was a clue that revealed the throne room is connected to the study room.
Evidence No.10 "Beastman Hair" was of Val's when he washed away the blood on his claws.
Details of Incident in Chapter 2
The only way to defeat the twisted human king Eida was to use Memory Wipe. Players can be made aware of how the nature of the "demon" works by looking at various hints and evidence provided in Chapter 2. It also has clues on how to wipe the "demon" away before it takes root in Eida.
Hints from Evidence
Experiment Report:
"xxxxx’s appearance changes when they’re inside the city. As the number of people who know their existence increases, so does their power."
"The presence of xxxxx spreads like wildfire through media exposure and electronic communication.
As a result, xxxxx’s power grew stronger and stronger."
From these writings, one can guess that the "demon" gains its power by being recognized by people.
A Researcher's Note: B :
"As expected, Magic Bomb didn’t even scratch xxxxx. The blast only slipped through his body. But the capital and the humans that got hit by the blast are not the same. The only one left standing is me. I'm all alone. As I lament how this continent is already beyond salvation, I noticed that xxxxx’s power had greatly weakened. And there, I realized something important. I then decided to take my own life."
From these writings, it may seems that the "demon" got sealed by having people got blasted by the Magic Bomb and the researcher's suicide action. But while their death is not exactly the direct cause, it results in people remembering the "demon"'s existence decreasing, making his power wane.
Other Hints
One can surmise that Eida wanted to link his name with his existence by how he always repeatedly called his own name when talking to others.
Additional Info
・Why did the races decide to join hands again?
This will be explained further in the after-story of Eida which you can buy in itohaki's BOOTH page.
You can find the links in the Afterword page.
But even by looking at the currently available character sheets alone, one can tell that each race has something that they lack and they're trying to fill that gaps in ways that they can. And later on, they realize that it's something they can manage if they cooperate well with other races. Although the memories of Eida had faded away, the fact that they had fought a powerful being together had etched into their memories. Thus, making barriers of distrust they have toward each other to slowly fade away.
・Isn't Eida's plan too much of a gamble?
Any visions of the future that Eida saw through "Foresight" ability is one that will always come true. While it's true that Eida himself doesn't know how it's going to be realized, he decided that it's a plan worth trying.
He also doesn't want to leave the "demon" as it is in Kataliah. Other than him wanting to help it, he fears that other people might make the deal with the "demon". For instance, if Val didn't meet Eida 10 years ago, he would still be a beastman hungry for power and might actually seek and make a deal with the "demon" instead.
・In the end, what exactly is the "Holy City of Kataliah"?
The Holy City of Kataliah was a kind of research city in the human kingdom in its heyday, where technology related to science and magic flourished. Eida was able to make potions of Memory Mend, the book about controlling magic, and magic barrier after he tried very hard learning for 10 years from several magic books that were left behind in that vacant city.