Border of the Afterlife

The is a region that lies between the living world and that of the. It is here that Tōga made his final resting place, and later where Hōsenki I attempted to hide the "final fragment" of the Sacred Jewel of Foul Souls. It can only be accessed through special gateways made by jewels such as the Black Pearl, the Gate of the Afterlife in the Realm of Fire that is guarded by Gozu and Mezu, or by following a stream of blood from large demons (e.g., Tekkei) who have a connection to the Netherworld even Sō'unga has power to transport to the border and open a gateway to the Netherworld. It is connected to the Realm of Fire, and there's another connection is from the Lake of the Staff of Two Heads.

Moroha's parents, Inuyasha and Kagome Higurashi were sealed there by Sesshōmaru for fourteen years. At one point, Moroha's spirit ended up there as a test set up by the strong barrier at Mount Musubi, as the quarter-demon girl had to chose what she wanted to protect and what she had to let go of. Glimpsing the gigantic skeleton of her grandfather, she wondered what it could be.

The strong demonic shield of Mount Musubi is connected as it allowed Moroha's spirit to travel there and get a glimpse of her parents whom she assumed dead. One month later, she and her twin half-cousins end up there during Sesshōmaru's showdown with Kirinmaru, where Inuyasha and Kagome finally reunite with their teenage daughter and meet their half-nieces for the first time in fourteen years since Kagome takes care of Rin.

Trivia

 * It is considered to a type of that exists between the immaterial worlds of,  and the Underworld, as Inuyasha and Kagome are revealed to be very much alive from having been incarcerated there for long fourteen years.
 * Time is implied to move very slowly in this realm as Kagome stated that she doesn't age much since she has been there even though 14 years has passed in the world of the living - proven by how she hasn't appeared to have aged since she was sealed there.