The Earth Relic Stone is buried in the King's Barrow in King Satrius's chamber. In order to get this stone, complete all the marked quests from chapter “Windbreak” (you can also follow White Head's questline for the same effect). Once you get the key from the mayor or White Head, you may get the stone froms the kings tomb. Be prepared to face a small force of regular and Ice skeletons, however. When you pick up the stone, the quest is solved.