This is story-related and will happen if you get through it all.
In the Southeast part of the Valley of the Kings, there are tombs near the Desecraters' Camp.
The story will have you doing each tomb in turn after several quests in between.
Tomb of Nefertiti takes you to Aaru map (level 45-47)
Tomb of Akhenaten takes you to Aten map (level 47-49)
Tomb of Ramesses takes you to Heb Sed map (level 49-51) (not required to get to Tut)
Tomb of Tutankhamun takes you to Duat map (level 53-55)
Each map will have you performing a specific set of tasks to make the Pharaoh appear for you to battle.
Ramesses is not required to get to Tutankhamun.
I killed Tutankhamun before Ramesses. It let me do this when I went after Isidora without doing the Ramesses quests. I was level 51 when I killed Tut.
confirmed by RobG88
you do not have to kill Ramesses. I took out Nefertiti, Akhenaten & Tutankhamun and the achievement popped
When you finally get to Tut, dispatch him in grand boss fashion for a nice achievement.
I use sword/shield because I like that particular overpower. Just dodge and strike and use the overpower when you can. He wields a big ass legendary spear, which you earn for defeating him. It's called Aten's End.
Be sure to equip or upgrade to level 55 weapons to give yourself more damage. Upgrading crafted gear helps greatly as well.
This video (not mine) shows what you can expect from each Pharaoh fight.