Complex sentence: During World War I he wrote war adventure novels, and war stories, some of which showed a strong return to his old weird style, such as "Marooned" in "Barbarians" (1917).
Split into simpler sentences: During World War I he wrote war adventure novels and war stories. Some of these showed a strong return to his old weird style, such as "Marooned" in "Barbarians" (1917).