After talking to Sinham, you'll have to find the three lost carved tablets. Fortunately, they're nearby and their locations are already marked in the map. The first one is right nearby, inside one of the ruined towers.
One is found inside the temple ahead. You may find relics (Spider 26) in the small altar along the way and under the water in the submerged room ahead (Shark 26), if you haven't found it earlier. The tablet is also on the floor of the submerged room.
The last one is inside the ruins near the coast. There's another relic here if you haven't gotten it yet. Climbing it up is a bit tricky at first. Just hug the walls to walk around them and to access it.
Talk to Sinham again to deliver the tablets and complete the mission.