Obviously you will have to choose a path near the camels. Talk to Mustapha along the way and listen to his complaints on being kicked out of his own house. Once this conversation has ended, go to your left and approach Herodotus (screen). Talk to the old man and you'll find out that you'll have to obtain pure water, so he'll be able to bless it. Don't forget to ask Herodotus about the sphinx and you'll find out more about the hieroglyphs. Wait for this conversation to end and go back to Mustapha. You should also check your inventory, because you were given one new item - a Host.
I assume that you've already obtained a mirror by using a diamond. Start off by choosing a casserole dish from your inventory. You've collected this item a long time ago, when you were here for the first time. You will have to fill the dish with water (screen).
Open your inventory once again and choose a casserdole dish filled with water. Look at my screen - that's where you'll have to use this item. Two final steps of this riddle will require you to use a mirror on the casserole dish and to combine the mirror with a flask. You may now go back to Herodotus. Give him the flask and wait for him to bless the water. A flask of Holy Water should appear in your inventory. Also, you're going to be transported back to the tomb.