Merging with Kohron did not bring Tenma the expected sweetness of peace and victory but a life of endless battle.
A Demon was an entity of anarchy, and a Demon Prince magnified this trait by a million times. Kohron was the Prince of Strife, his essence craved disorder, for without it, he would wither away. His soul might not be present, but his body was an extension of his soul, and this was the body that Tenma had possessed.
Tenma was just a High god, powerful when placed on a galactic scale, even a power to watch out for when placed against a universal scale, but the heights that a Demon Prince stood at meant he could as well be an ant before a mountain.
The body of Kohron in this universe was just one of many places in many different universes where the dominion of the Great Abyss held sway. Their bodies and essence were nothing that a god could conceptualize.