Complex sentence: Mafuyu has been missing for two weeks, after visiting Himuro Mansion to look for his mentor, Junsei Takamine, who had also gone missing in the mansion with his assistant and editor while conducting research for a new novel.
Split into simpler sentences: Mafuyu has been missing for two weeks, after visiting Himuro Mansion to look for his mentor, Junsei Takamine. He had also gone missing in the mansion, together with his assistant and editor, while conducting research for a new novel.