Non spoiler versionThis is story-related and only appears after you complete:
"The Land of Turquoise" - level 38
"Where the Slaves Die" - level 39
"The Walls of the Ruler" - level 41
"The Setting Sun" - level 43
"No Chains Too Thick" - level 43
"Sic Semper Tyrannis" - level 45
The final mission is "The Greater Good" - level 45
Minor spoilers version (with video)*** Spoiler - click to reveal *** When you meet your Hidden brothers in Sinai and learn about General Rufio and his 3 evil underlings, you'll have to assassinate those 3 to get to Rufio:
Tacito - "Where the Slaves Die" - level 39
Ampelius - "The Walls of the Ruler" - level 41
Ptahmose - "The Setting Sun" - level 43
They will net you this other achievement
Then, you must complete "No Chains Too Thick" where you meet with your Bureau and plan your hit on Rufio and some subsequent extra stuff I won't spoil.
Then you must complete "Sic Semper Tyrannis" where you finally take out Rufio.
Then you get the mission "The Greater Good" where you must have a 'little chat' with Gamilat.
courtesy of Mad foX on YouTube