Each region is England has a primary story called an Arc.
The Suthsexe Arc is around the 10th Arc you complete in England. You have some flexibility in the order you complete the Arcs.
The Suthsexe Arc consists of the following missions:
Reaver of the South
Let Them Eat Ashes
Severing the Lines
Storming the Walls
Of All That Has Passed
Reporting on Suthsexe
These missions collectively take around 2-3 hours to complete.
To give some perspective of when you can expect this achievement in the game: Upon completion, I was at 62 hours played and 57% completion. You can find these stats on Ubisoft Connect.