Grim Comet

"A great catalyst is approaching."

- Riku

Every 500 years, the Grim Comet would travel close to Earth. War and famine would spread the land, causing the deaths of many people. Since the human population was declining at the time, the yōkai population was on the rise and this made powerful two  Daiyōkai control them in the East and West. One being Tōga, ruler of the West, and the other, Kirinmaru, ruler of the East. The two beast kings joined forces and destroyed a fragment of the comet.

500 years later, the Grim Comet traveled close to earth once again, and because Tōga is no more, and Kirinmaru has fallen into a long slumber, the sons of the Great Dog Demon joined forces and destroyed the large fragment of the comet with Sesshōmaru's Bakusaiga and Inuyasha's Meidō Zangetsuha

Trivia

 * Based on the loop the comet is on, it likely means the Modern Era will be facing the comet's return. As it seems demonic entities reside in the comet, it implies Modern weapons have no chance at destroying the Grim Comet; which would explain why Sesshomaru "saved" Inuyasha; he noted Meidō Zangetsuha would still be needed at a later time.