Hint: At the Pod Landing Site, the H-shaped area on the map, it is located in the lower left area of the H. There is a passage that leads to King Agni's Tomb. Once at the location, the Tablet Seal must be investigated and a door will slide open. The quest objective (the orange exclamation point) can be seen from the Valak Pass.
"Investigate the inside of King Agni's Tomb."
Hint: LV46 Brutal Gravar guards the coffin. It must be defeated in order for the quest to progress further. After its defeat, the coffin must be inspected.