Gehaldirah sought to achieve reincarnation and he succeeded. It is with his understanding of life that he repurposed the artifact for punishing transcendents into separating the body of a Sovereign. A complete body of law of a Sovereign is vastly different from a soul body of a transcendent. It is a feat on its own to destroy the body without the soul.
Without life essence, his soul wouldn't have been able to heal when he destroyed his body to separate it from his soul. That feat alone of becoming an origin god with just his soul is the foundation for his reincarnation. It created a deficit in his existence that the universe sought to fill up with bodies.
His knowledge of arrays allowed him to create a formation capable of teleporting his soul into the law matrix by using the seed of a Sovereign as the key to it and other body parts as the destination within the law matrix.
Everything worked to lead to what happened. When parts of his soul were thrown into the law matrix, the universe tried to tie them down with bodies for them based on the items he used as the destinations for the souls. Stelios has chosen Helios to be the destination for his consciousness.
Stelios is also unique in that he has a lot of time to develop his knowledge of runes. He has been the ancient sun god for numerous Origin cycles. He is older than Gehaldirah by at least 200 times. Gehaldirah was 5 Origin cycles old. Stelios is at least 1000 Origin cycles old and has been stuck in his position for that long. His hopes of progress had all but died until he came across Helios.
Stelios noticed a lot about Helios when he was a fetus. He noticed that Helios's soul has an inherent disconnection from his body as if he is a visitor wielding the body instead of a homeowner borne within the body. There's a gap between the body and the soul that created a sort of dissonance between them. He was able to get this information through the divine energy that he helped Helios with.
He didn't think much about how the dissonance came to be. According to his observations, it likely had something to do with the Origin energy wafting around Helios's soul. It is not uncommon. It is how Colossi are created. It just so happened that Helios's soul had an extreme reaction to the exposure of origin energy.
The reason why some transcendents become Colossi instead of Behemoths is a well-understood phenomenon and of no interest to him. He focused more on the effect of the dissonance. The dissonance between body and soul is why Helios refused the encroachment of the fragments of order. Stelios planned there and there to take advantage of that dissonance. He created a Stigmata for that very purpose and gifted it to Helios.
One function of the Stigmata is to give more divine energy to Helios. The other function will only activate when Helios is trying to become a transcendent. It is why Helios didn't notice the other more insidious function of the Stigmata when he examined it.
Helios accumulated a lot of divine energy and reached his limit, Stelios took control of the divine power through their connection to stop the process of fusion. The divine energy formed a wedge between the body and the soul. The dissonance between Helios's body and soul allowed a small space to be present between them for the divine energy to enter. It is like a wedge that he can take advantage of.
"I did it. Now to take my reward." Stelios laughed maniacally.
First, he tested the destination. It is a mere precaution. He doesn't expect anything bad to happen but one can't be too careful. His divine Will resonated with the divine will within the Stigmata. He found his divine will to be intact, indicating that it has not been tampered with. It is the go-ahead he needed.
Then he forcefully descended through the connection of the channel into the wedge. There was no friction or resistance. The channel between them through the Stigmata had been expanded. It allowed a lot of divine energy to go through and now is allowing Stelios to go through with his Godhood of the sun god.
When he reached the wedge he said to Helios. "Sorry about this boy. You were betrayed by the other gods and now you will be betrayed by your divine father. It's nothing personal. I'm just taking my due since I saved your life before you were born. You owe me your life and now I am taking it."
Helios didn't speak. Apparently, he was too stunned to talk. So Stelios continued talking to distract the boy so that he can take over his body and complete the breakthrough in his place without any resistance. It's not like any resistance will work against him. He is a grand god after all. What can a weak mana entity do to stop him?
"I'm sure you must be shocked by what is going on. But I have more to say. I'll tell you this so that you can understand why I did this." Stelios paused to create suspense. Then he said, "I am not your father."
Stelios continued to laugh as Helios wallowed in shock.
"Don't worry. I won't kill you. You can have my Godhood when I take your body. Isn't that great? You will become the next god of the sun. All your divine siblings will be so jealous."
He is not out to kill Helios. He will possess Helios's body. Helios will have to bind with the Godhood of the sun god if he wants to live. It is more of a swap than a possession. They are just swapping positions. He will take Helios's body and his identity while Helios becomes the sun god. That what's he was raised for after all. It's not that he doesn't want to kill Helios. It is that he can't, at least not in this form. It is only right that the son of the sun becomes the true son.