Egypt has gone too far! They think they're so great with their pyramids! Well WE can build pyramids too!
In this mission, you play the Nubians. The Nubian King has instructed you to build a Grand Stepped Pyramid Complex at a site in the mountains. He also wants a Small Stepped Pyramid. He also wants a Large Obelisk to proclaim Nubia's glory and to let everybody know what a bunch of losers those Egyptians are.
Resources are very scarce. The Nubian King did not really select the site of this new city with an eye towards habitability. You are going to have to trade for nearly every item your citizens will need (or, at least, the raw materials to make them).
The Nubian King chose this site because it is on a river. He imagines that this river is like the mighty Nile! It's not. The river is small and turgid, with barely enough room for fishing boats, much less anything larger.
It does have seasonal floods, but they are short and poor and cannot be counted upon to provide much in the way of soil enrichment. Not that there's much room to farm anything on the floodplains.
You've heard of the god "Osiris" worshipped by the Egyptians. Maybe the Egyptians are onto something. Maybe he might be worth supplicating. Maybe HE might make the floodplains more fertile.
Just don't let word get back to the Nubian King.
By the way, the Egyptians have gotten wind of the King's plans. The Egyptian Pharaoh sent a Cease and Desist letter to the Nubian King, claiming that "Pyramids(tm)" were the intellectual property of the Egyptians, and that the Nubians couldn't build one of their own without prior written authorization from the Pharaoh himself.
The Nubian King told the Pharaoh to get bent (only he said it using fancy ROYAL language), so you can expect the occasional invasion from well-armed Egyptian lawyers seeking to protect their trademark.