Final story related spoiler! Well done guys you made it! PREPARE TO BE SPOILED!
*** Spoiler - click to reveal *** First thing, this mission takes place on the 15th March 44 BC in Rome. For all you history buffs, you know what's about to happen.
After killing Flavius you go through some cut-scenes, and begin play again as Aya on a ship sailing for Rome. Destroy the enemy fleet (ends with twin Octaremes) then you proceed onto Septimius.
For what is essentially a final boss, this guy is such a wet-blanket. His attacks hardly hurt,
and he doesn't take all that much of a beating to put him down.
After this you must reach the Senate chamber where Caesar is addressing the senate. This is a bit harder, don't just run at it like a mad-man (like yours truly). Try as best as you can to sneak around. There are dozens of guards in this area, but you can fight them in groups of two or three if you are so inclined.
When you are no longer in combat (If you got into it in the first place) you enter the senate chamber and assassinate Julius Caesar. Well that AC game set in Ancient Rome probably won't happen now. So many dashed dreams.
After you go through the final cut-scenes and regain control of Bayek, the achievement will be yours!
Let's keep cheevin' together!