The clue Karna searched for was the requirement to completely control a domain. He had nothing among the historical records to learn about it since it had never been done before. The only thing he had was his investigation of the Ankseram curse, which led him to believe that someone was trying to control the domain of death.
He didn't find anything at first, and he went on with his life while keeping it in the back of his mind—until Cana confronted him about dealing with dangerous situations without asking for help from others. At the time, he was feeling a bit disappointed about how worried he was making Cana with the decisions he had made.
But later on, something came to his mind. As he was flying with the Frostborn Lacrima, he had discussed many things with him, along with the fact that he had gotten some information from someone from the future who had talked to him about it when he was young. But as he spoke, a sudden realisation came to him.
"Lucy from the destroyed timeline and Fallen Rogue. Why wasn't the Lumen Histoire used in that timeline where everyone was dead? Zeref would have been alive no matter what the dragons did. Even Mavis's spirit would have been alive as well. Why didn't she tell anyone about that power?" Karna spoke to himself. He knew that hundreds of dragons from the Eclipse Gate had hit the whole of Fiore in that shattered dimension.
He also knew that the last two people of Fairy Tail left were Levy and Lucy. But there were strong enough Fairy Tail members who might not have survived a fight with a dragon but would have had enough power to at least run away.
Makarov would have even released the memory seal on Doranbolt to at least teleport Wendy and the other young people he could get his hands on, while he gave his own life to hold back the horde of dragons. With Makarov's character, Karna was more than sure that Makarov would have used Fairy Law at its fullest extent, even sacrificing his own life to inflict as much damage as he could to save his children from the dragons—at least to make sure they had enough time to escape.
But Karna was also sure about one other thing. Makarov was smart—smarter than he ever let anyone realise around him. He might have given the impression of a youth who was headstrong in his younger days, but as he took on the title of Master, he had begun to change, though still hiding it behind a drunken, grumpy old man persona. Makarov was the one who realised the war with the Alvarez continent was not over and had placed Doranbolt in the council as a mole for more than a decade so that he could be prepared for any retaliation.
Facing hundreds of dragons, Karna was sure Makarov would have realised that Lumen Histoire was the only hope they had, and he would have made sure that it was used. Makarov was a mage type who had experience with a lot of magic, including memory magic as well. He would have used it while fighting to send relevant information to the people to use Lumen Histoire. And let's not forget Gildarts, who also knew about Lumen Histoire.
Now, even if by some really big coincidence, Makarov was instantly killed by the dragons or Gildarts was away by the time he knew and was killed by the dragons, that still leaves Mavis and Zeref, who are practically immortal due to the curse.
The Shadow Rogue had come from a future many years later than the time future Lucy had come from, and according to him, dragons had destroyed most of the world and Acnologia had destroyed everything, including the rest. He never mentioned the killing of the hundreds of dragons that came first, and he didn't know about the Lumen Histoire. And if he did, that would have been his first target as soon as he came to this dimension.
So, everything leads to one thing alone—Zeref, Mavis, and the Lumen Histoire completely disappeared from that timeline.
"Exactly like it did in the canon," Karna spoke in the present.
"So, I ask again, Karna, what is the accursed human god aiming at, and how did he accomplish it?" Jason asked with a wider, expectant smile, and Karna replied:
"To gain total control of the domain of death, Ankhseram is planning on using Lumen Histoire, the infinite magic, as a form of sacrifice. And to do this, he requires both the bodies of Zeref and Mavis at the same time. His curse on both of them will be strong enough for him to steal that magic at the opportune time." his fist clenched as he said the rest.
"In the canon, he manipulated Zeref's mind to start the war and had used the feelings of both Zeref and Mavis to his own gain. At the time when they were prepared to die together, he used the opportunity to take up all the magic onto himself. That is why they did not die the first time they loved each other, and Mavis had been turned into a spirit. He didn't want them to die so soon, not until he was sure they had managed to create something he could use as a sacrifice.
Both Zeref and Mavis were chosen not only because they touched upon the domain of death, but also because of their intelligence. As long as he kept them alive and suffering, he was sure that they would create something that could be used to take control of the entire domain of death—and they did.
This is a complete guess of mine, but all of these hypotheses can be confirmed by seeing whether Nirvana—the closest thing to show similar powers to the curse of Ankhseram—was somehow created by the same person who touched upon the death domain.
And from the records Cana collected, he was among the people who had created Nirvana. But he was not a scientist, only a supervisor of those researchers.
He has experience with controlling and ordering smart people to do his work. And if I am right, he started from Nirvana. Right?
Even if the price for turning evil to good required balance, they would not have made a mistake that would have led to the citizens of Nirvana being affected by it.
He diverted the negative feelings to the city, killed all the inhabitants, and used it as a sacrifice to gain control of the majority of the death domain.
Tell me, Jason, tell me. Am I right?" Karna asked his question at the end.
A/N- hope you like it.