Here is the full list of all 16 Downward Spiral: Horus Station achievements.
Complete Act 1 on any difficulty.
Complete Act 2 on any difficulty
Complete Act 3 on any difficulty
Complete Act 4 on any difficulty
Complete Act 5 on any difficulty
Complete Act 6 on any difficulty
Complete Act 7 on any difficulty
Completed Downward Spiral: Horus Station
Chose Explorer mode and completed an Act
Completed an Act on Oppressive mode
Found one collectable in Horus Station
Found ten collectables in Horus Station
Found all collectables in Horus Station
Completed an Act in Co-Op
Played ten rounds of multiplayer in Horus Station
Grabbed every international item in Horus Station