Ultraman Tiga's Final Battle

Strange things happened one after another, and the long-lasting tranquility of the earth was once again broken. Yuan Yi encountered abnormal phenomena and monsters during the assessment process, and was finally accidentally brought to 3,000 years ago. Some ancestors predicted: "When the dark demon descends again, it means the coming of a catastrophe... This catastrophe is both man-made and human." What exactly is this catastrophe? Since it is man-made, who made it? None of this is known... Facing the Kirialod representatives from hell, what interesting things will happen between them and humans? Is it war or peace? Is Masaki Keigo necessarily evil? What happened during the time he disappeared?