King Agni's Tomb is a Secret Area in Xenoblade Chronicles. It is located in Makna Forest, near the Pod Landing Site, and southwest from Valak Pass. Initially, the tomb is sealed, preventing the party from going further than the entrance.
However, a line of quests from Lupa — a Nopon in Frontier Village who can be found around the metal hut on the Archaeology Level — allows access to the inside of the tomb. She is interested in the tomb and the Giants, thinking there is some sort of connection, and she sends the party to check it out for her. Three Ancient Documents must be obtained to open the tomb, which is sealed.
Once the tomb is opened, the doors open and it reveals a short corridor with level 40 Buio Vangs and Makna Antols. The corridor opens into a large hall laden with dead Kromars; most likely the result of a battle. At the end of the hall is Brutal Gravar, a Unique Monster, in front of a large stone coffin, the Agni's Coffin.
Once it is defeated, the Agni's Coffin cannot be opened. The party must report back to Lupa, then gather ingredients for a lubricant oil to open the coffin. Once that is accomplished, the party must return, Brutal Gravar may respawn. By opening the Agni's Coffin, a small passage appears and leads to a small room with a Giants' Treasure Chest. The contents of the Giants' Treasure Chest can then be taken, which includes the "Eater" Records and an advanced art book for Battle Soul.