The Inu no Taishō's tomb, also known as the grave of Inuyasha and Sesshōmaru's father, was not really a grave at all, but rather the giant skeletal remains of Inu no Taishō. The gateway to his tomb was hidden in a black pearl, which was inside of Inuyasha's eye. The location was kept hidden from his oldest, full demon son Sesshōmaru, and at the beginning of the series, we were introduced to Sesshōmaru as he hunted for the location of his father's grave; the Tessaiga was kept here.
The Inu no Taishō's tomb took the form of an enormous dog-like skeleton with armor on the shoulders and on the chest. According to Myōga, this was his true form.
Inu no Taishō's tomb was on an alternate plane of existence from the world of the living, and thus could not be accessed in a traditional way such as by flight or on foot. All of the known entrances to the grave had quasi supernatural qualities. The original pathway was a black pearl, a gem that acted as a direct gateway to the tomb, and it was placed within Inuyasha's eye. The other known methods of traveling to the grave were by entering the gate at the Realm of fire, a feat which only Sesshōmaru was able to accomplish due to the Tenseiga. The other known method was concocted by Naraku. Tekkei, a demon associated with the afterlife, was beheaded, and the river of blood that flowed from her neck created a pathway to the tomb. Though this path opened and closed once the blood dried fairly quickly, also seemingly leaving Inuyasha & group stranded.