Assimilating the Essence of a Demon Lord was not easy, especially one that was as broken and tortured as Tenebris, in fact, it should have been impossible, but Rowan and hence his Reflection were a weird and unique beast because he believed that there was nothing in existence that he could not see through or master, his foundations were simply too potent, and every evolution he went through made his potential to surge to heights that were almost impossible to be measured.
Although his ability to master everything was true in many cases, there was always a limit, but his attitude and willpower had made that limit extremely malleable. He could push past what should be possible even with his perverse talent because he simply refused to believe that he was not capable enough not to achieve what he had set his mind towards.