Black Pearl

"Seeing, but never seen; protected, yet never known to its protector. Our father's tomb, hidden inside a black pearl deep within your eye."

- Sesshōmaru

The is a demon relic which provides its bearer with a gateway to "the other side".

Inuyasha
Knowing that his elder son Sesshōmaru would actively seek the Tessaiga following his death, Tōga decided to keep the sword with his remains in the other world — where his firstborn could not follow — until Inuyasha was able to use it himself. In order to provide Inuyasha with access to the other world, and thus the sword, he commissioned the demon Hōsenki I to create a jewel that could open a gateway there. Once forged, Tōga had Myōga to place the Tessaiga in his master's domains and the path within the pearl, or behind, Inuyasha's right eye.

Some 200 years later, Sesshōmaru finally tracked down the pearl's location after decades of searching and claimed it forcibly from his younger brother's eye. He used it with Nintōjō to open the portal to his father's resting place and passed through it, with Inuyasha, Myōga and Kagome following him. Once the sword was claimed, the characters returned to the world of the living through the same portal. After closing the way, the pearl returned to Inuyasha's eye and became a permanent part of him.

In the later seasons and chapters, the characters discovered that a jewel shard had somehow made its way to the netherworld. After asking how to use the pearl to reach the graveyard again, Tōtōsai and Myōga informed Inuyasha that by providing him access to his inheritance, the pearl had served its function and could no longer link them to the netherworld.

Hanyō no Yashahime
Hōsenki's son and successor, Hōsenki II reveals that the pearl was originally formed from the tears shed by Inuyasha's mother, Izayoi when her beloved husband passed away. Using the power of the green Rainbow Pearl and Izayoi's deep sadness contained in the Beni given to her by Tōga, Hōsenki II was able to produce a new black pearl in just 4 years and gave it to Inuyasha, who sealed it in his right eye.

Sesshōmaru later extracted the black pearl from Inuyasha's eye when he and Kirinmaru confronted him and Kagome Higurashi and used it to save them from Kirinmaru's wrath by sealing them inside Tōga's tomb, with it being said that they will eventually be released as Inuyasha's Meidō Zangetsuha would be needed in the future.

It is most likely in the possession of Sesshōmaru to keep it safe.

Manga
In order to extract the Pearl, Sesshōmaru gouged out Inuyasha's right eye, which slowly recovered, over the course of the next few chapters. In the anime, this was modified, so that Sesshōmaru simply used a stream of energy to extract the pearl, without damaging the eye itself, though the pupil did temporarily disappear.

Anime
The Pearl returning to Inuyasha's eye is exclusive to the anime. In the manga, the group's return was never really shown, and its fate was not revealed until the group tried to get another from Hōsenki where it was revealed that it had simply vanished into thin air, upon completing its purpose.

Physical description
It is a small pearl of deep black color like polished onyx and of a spherical shape of the type of the Pearl of Shikon. This pearl possesses demonic powers and its portal function is unique and inalienable.

Demonic Properties and Skills

 * Portal: It is able to create a unique portal that leads only once to a specific place. In this case, it accesses the Great Dog Demon's tomb.

Trivia

 * The Black Pearl is so potent a relic that it takes enormous skill and possibly decades to forge. Hōsenki's son explained it would take roughly 100 years for him to master the process of making one.
 * When Inuyasha returned to his father's grave for the second time, the Black Pearl was somehow able to give him visions of his troubled past as a way to warn him that he was not meant to go there a second time.
 * Sesshōmaru's search for the Black Pearl is what got Jaken his job. The demon weapon Nintōjō had the ability to track down the pearl, but Sesshōmaru was unable to use it himself. When Jaken proved capable of tapping into the power, he was allowed to accompany the daiyōkai as his servant.
 * The Black Pearl looks much like a tainted Sacred Jewel.
 * In Hanyō no Yashahime, a set of pearls possessing varied great demonic powers which they bestow onto their wielder, known as the fabled Rainbow Pearls, were created. Only the silver Pearl displayed the spatial ability to open a portal to the distant future.