You must complete the mission The Man Who Would Be Jack to unlock this achievement.
*** Spoiler - click to reveal ***
The Man Who Would Be Jack is the 14th mission of the Borderlands 2 main story line.
Mission List in Order:
My First Gun
Blindsided
Cleaning Up the Berg
Best Minion Ever
The Road to Sanctuary
Plan B
Hunting the Firehawk
A Dam Fine Rescue
A Train to Catch
Rising Action
Bright Lights, Flying City
Wildlife Preservation
The Once and Future Slab
The Man Who Would Be Jack
Where Angels Fear to Tread
Where Angels Fear to Tread Part 2
Toil and Trouble
Data Mining
The Talon of God
Ending