Grim Comet

"To warn you a great cataclysm is approaching. Every 500 years, an object known as the Grim Comet travels close to this earth. Unfortunately, large fragments start to break off and descend from its core."

- Riku

The Grim Comet is a demonic asteroid from outer space that is full of demons inside. 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 two powerful Daiyōkai control them in the East and West. One being Tōga, ruler of the West, and the other, Kirinmaru, ruler of the East. First in 1005 AD (Heian jidai), the two beast kings joined forces and came up to the palace of Sesshōmaru's mother's until later that evening as the Grim Comet fragment approaches, they destroyed it by Kirinmaru's Blasting Barrage and Tōga's Meidō Zangetsuha. 500 years later in the Sengoku jidai, 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 came to same place in the right spot in the right time, they destroyed the large fragment of the comet with Sesshōmaru's Bakusaiga and Inuyasha's Meidō Zangetsuha.

14 years later in the feudal era, Kirinmaru spotted the Grim Comet as 500 years later by the Reiwa Era, the Grim Comet appears again. Noticed by Osamu Kirin, he takes notice that humans such as Sōta Higurashi cannot perceive the comet in the modern era. Even though he felt that he had a connection with Kirinmaru between two eras in which that he stated that the Grim Comet "does appear in the other era".

Trivia

 * It seems the comet can't be detected by modern technology, as there was no broadcast announcing a "new" comet being spotted in the sky.
 * As no powerful demons exist in the modern era, and with the portal in the Bone-Eater's Well shut, there's no way to get Towa, Setsuna, Moroha, Sesshōmaru or Inuyasha there to help even if he was freed from the Black Pearl.