Obtain Quest: Head back to the Sandy Shack, there should be a woman standing in front of it. Talk to her and the proceed to the palace
Prerequisites (As Far As I’m Aware): Complete Nice to Meat You
Guide
Head back to the Sandy Shack, and go to the door. A woman standing opposite (Soi-Fon) will talk to you, and after another conversation, head back to the palace. Go inside, and up two floors. There you'll have a conversation again about the leader's graverobbing brother (the red-haired man in front of the inn), and be asked to help him. Leave, and after another short conversation with Nazir, head over to the red-haired man (can't remember his name (it's Andrew), so let's just call him Red for short from here on).
Talk to him, and he'll complain he's thirsty. Go into the bar and talk to the still cook, and he'll give you water. Take it to Red, and he'll throw it on the ground. Go back inside and talk to the cook again, and you'll get the option to buy a drink for 10g. Give that to Red, and he'll ask you to collect 4 scorpion tails. Go out into the eastern zone again, and fight scorpions until you have enough tails - you can get 1 or 2 from the fights, depending.
Bring the tails back to the man, and he'll tell you about the tomb. Follow him, and eventually you'll come to an area blocked off by a boulder. You'll be asked to find another way. Go back into the previous zone to the west, and interact with the quicksand pool. You'll end up in a room with 5 coffins. 2 of them contain mummies to fight, 1 contains some gold, 1 contains the tablet you've been looking for, and 1 has the key to get out of the room.
Once you have them all, leave through the south door, opening the statue in front of the door with the key to open the door itself. Beyond that, you encounter Ishizu Ishtar, who takes the tablet and fights you. She has three forms that you'll have to fight in a row. If you do lose at any point, it won't give you game over, but you'll have to walk back into town to heal up unless you have any phoenix downs on you. On the way back, you'll pass Red, who, if you don't have the tablet, will leave to wait at the tavern.
Once you defeat Ishizu, you take the tablet back and take her as a slave. Take the tablet back into town and give it to Red, who'll offer to teach one of your characters the Holy attack spell for 500g. To finish this quest, you have to have a character learn it (white mage is the only one I've confirmed can learn it), so you may need to wait a few days to get a slave capable of learning it if you don't have one or can't yourself. Or just take the 500g he offers.