Complex sentence: He became obsessed with the Chapter Black video tape, which documented the worst cruelties of the human race against demons and each other, and he eventually disappeared, deciding to punish humanity by opening a stable tunnel to the demon plane.
Split into simpler sentences: He became obsessed with the Chapter Black video tape, which documented the worst cruelties of the human race against demons and each other, and he eventually disappeared. He eventually decided to punish humanity by opening a stable tunnel to the demon plane.