Complex sentence: In his quest for immortality, he tried to use Köinzell due to his human-fairy nature, but his callous attitude to relizing who Koinzell was caused Koinzell to go berserk and mutate into a more powerful winged form.
Split into simpler sentences: In his quest for immortality, he tried to use Köinzell due to his human-fairy nature. But his callous attitude after realizing who Koinzell was caused Koinzell to go berserk and mutate into a more powerful winged form.