Theory time, so what if after losing Sajun, Kael made contract with some demon/demon king to find Sajun again, that's why his current curse has Sajun's mana in it. Because he needed some kind of link to connect with the current Sajun in this world. Also, the demon managed to cross over because it's connected to Kael's soul. I do wonder if devouring his soul has a bigger meaning to the plot or just a means to an end
Theory time, so what if after losing Sajun, Kael made contract with some demon/demon king to find Sajun again, that's why his current curse has Sajun's mana in it. Because he needed some kind of link to connect with the current Sajun in this world. Also, the demon managed to cross over because it's connected to Kael's soul. I do wonder if devouring his soul has a bigger meaning to the plot or just a means to an end