The Tomb of Rakkis is the burial site of King Rakkis, located in the Ruins of Corvus below the Blood Marsh of Westmarch. The location of the Tomb was unknown to most humans, according to Diadra the Scholar. Tyrael and the New Horadrim attempted to hide the Black Soulstone there after Diablo's defeat, but Malthael attacked them and took the stone for himself.
The Tomb is not accessible during gameplay, but is depicted in the opening cutscene of Diablo III: Reaper of Souls.