The Realm of Fire (火の国, "Hinokoku") is a volcanic area. It contained a gateway that lead to the Border of the Afterlife that was guarded by Gozu and Mezu. Inuyasha and the others traveled there to try and get to the "last" Sacred Jewel fragment before Naraku or Hakudōshi could. They were met by the rock statues who immediately came to life and attacked Inuyasha, as only those who have died can pass through. Kagura of the Wind secretly followed per Hakudōshi's orders, but was shocked to see that all who entered recklessly would become statures instantaneously.
Eighteen years later, we see the Realm of Fire and its rock guardians again when the three half-demon princesses, along with Inuyasha and Kagome Higurashi, are trying to find a way to return to the world of the living. Inuyasha later realizes that Towa can use Tenseiga to open the doorway between it and the physical world. Shippō goes there to be there for his best friends when they finally are released.