Gameplay Walkthrough...

Complete the indicated task to unlock the achievement.

Achievement	How to unlock
A Complete Nightmare	Complete the campaign on Nightmare difficulty.
Been There, Done That	Find all Discoveries and Locations in the city.
Break On Through	Complete Act 1.
Change Of Heart	Complete Act 3.
Clear Skies	Take down the Rifter dome over the city.
Déjà Vu	Complete the prologue.
Endless Possibilities	Find all Weapon Mods in the city.
Key to the City	Earn a Gold Tier Thorn Stash key.
Overstuffed Pockets	Find all Weapons and Gear in the city.
The High Ground	Complete Act 2.
What's Behind Door #1	Gain access to your first Thorn Stash.