This Assassin's Creed Origins walkthrough will help you out with the New Kid in Town quest, which eventually involves searching for the Temple of a Million Years.
Finding the Temple of a Million Years in Assassin's Creed Origins is one of the coolest moments. To experience this you'll need to pick up a quest called The New Kid in Town quest, which is available in Letopolis, a city in Sapi-Res Nome, to the east of Lake Mareotis.
The New Kid in Town quest
The New Kid in Town is a level 18 side quest you should spot on your map. Upon arrival in Letopolis, speak to Ramessu, the orator on the platform west of the temple. He'll direct you to Nehi, the titular new kid.
Nehi sends you to fetch his ox cart. Use Senu to spot it, then battle or evade the bandits before driving it back to Nehi's house in Letopolis.
After speaking to Nehi, go inside his house and examine the writing on the wall. This will grant a new objective to speak to Ramessu, who suggests you talk to other citizens.
Use Senu to spot three home owners. Approach each in turn and, after speaking with them, go inside their houses and examine the writing on the wall. This will give you the full text, and grant you the objective to find the Temple of a Million Years.
Find the Temple of a Million Years
The Temple of a Million Years is an unmarked location, but pretty easy to find as it's very close by. It's a short walk outside Letopolis, heading west. Just stroll into the dunes and fly around as Senu until you spot the rows of lion statues that mark the entrance.
There are a number of bandits and a lion roaming around the Temple of a Million Years area so be careful on approach. To get inside, just walk right up the centre of the structure, between the final two lion statues, and look for a narrow crack in the stone wall to push through, right where you'd expect the door to be.
The temple has a pretty straight forward layout so the only thing you need to worry about are the snakes in the amphorae around the area; if you smash something, be prepared to fight the hissing inhabitants.
Hold up on the D-pad to highlight some loot as well as four clues to the mystery of the Temple of a Million Years - a statue of Sekhmet, a jug, a prayer and some skeletons.
Having done all that, Bayek solves the mystery of the Temple of a Million Years in a short cutscene, and you'll be directed to return to speak to Nehi one more time to complete The New Kid in Town quest.