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. 500 years ago, back when nobles were still governing this land, war and famine continued to spread, costing many their lives. Because of that, the demon race grew in numbers, with powerful demons who would rule them appearing in the East and West. You know one of them quite well, the Great Dog Demon. Him and Kirinmaru...these two Beast Kings joined forces and destroyed the Comet's fragment 500 years ago."

- Riku

The is a demonic asteroid from outer space that is full of demons inside.

In the Time of the Beast Kings
Every 500 years, the Grim Comet would travel close to and a fragment would fall. 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 and. One being Tōga, ruler of the West, and the other, Kirinmaru, ruler of the East. First in 1005 AD, the two Beast Kings joined forces and came up to the palace of Sesshōmaru's mother's until later that evening since the Grim Comet fragment entered the and approaches to the palace, they destroyed it by Kirinmaru's Blasting Barrage and Tōga's Meidō Zangetsuha.

Return in Both Eras
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's destructive ability and Inuyasha's Meidō Zangetsuha.

Fourteen years later in the feudal era, Kirinmaru spotted the Grim Comet 500 years later in the through his second incarnation Osamu Kirin, who takes notice that humans such as Sōta Higurashi cannot perceive the comet in the modern era. The Grim Comet would inevitably collide with Earth, indicating that the humans will become demon fodder and become nonexistent, ending in apocalypse of the degenerate age which the Tree of Ages intends to have the Half-Demon Princesses and their parents prevent at any cost.

Osamu would get rid of various yōkais that emerged from several fragments of the approaching comet that fell on Earth including Shibugarasus, Toad yōkais and Kumogashiras, and convince the Half-Demon Princesses who made their way back to the modern era through the Windmill of time to cooperate with him. After they get rid of the Kumogashira at Yokohame Arona, Osamu and the Half-Demon Princesses head to the top of, and prepare to destroy it once and for all before Kirinmaru does.

Osamu flew into the Grim Comet and noted the powerful miasma that would kill any humans who breathed it in. He also felt the mass of demonic energy and eventually felt the comet's core. He told Setsuna and Towa to destroy the miasma around the comet. Then, Moroha would purify the demonic energy, leaning the comet itself exposed. The girls would then combine their attacks to shrink it and Osamu would stop it himself.

However, when the girls attacked the comet itself, they couldn't even cause it to shrink and the demon energy returned. Towa then used the Zanseiken to absorb some of the demonic energy, and threw the demonic/stellar energy back at the comet tenfold, causing it to shrink. Upon seeing a pale blue glowing substance deep within its center, Osamu was then able to stop the comet and took it back with him to the feudal era after entering through the portal Kirinmaru was opening. The comet shattered through time and space and summoned a horde of demons with it. Osamu told Kirinmaru that he brought back the Grim Comet to Kirinmaru, so he alone could destroy it with his Bakuseiken.

Manga
In Chapter 18, it is told that the Grim Comet is a manifestation of Amaterasu's discarded malice.

Trivia

 * With the introduction of this comet, and the fact it brings demons from outer space, it is heavily implied that at least some demons are aliens.
 * The comet approaches every 500 years than the ordinary s approaches every 470 years according to Osamu Kirin.
 * 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.
 * In Episodes 39, 40, 41, 43, and 44, the Grim Comet's tail's color goes to rainbow than red in episodes 15, 22, and 23.
 * Later in Episode 41, Grandpa Higurashi seemed to detect the Grim Comet's presence in the sky, even though he cannot actually see it.
 * Grandpa sensed that there are disturbing shadows from the trunk of the Sacred Tree, which this is an ominous sign, and something even "worse" might be approaching was the Grim Comet.
 * The comet can emit powerful waves of miasma in Episode 44; its true form seems to an entity called the Grim Butterfly, which has been lying dormant deep within its center.