Complex sentence: She was considered to accompany Momonga in his exploration because she looks very human, but he decided on Narberal instead, something he regretted after witnessing Narberal's coldness toward other humans.
Split into simpler sentences: She was considered to accompany Momonga in his exploration because she looks very human, but he decided on Narberal instead. Which is something that he regrets after witnessing Narberal's coldness toward other humans.