Here is the full list of all 21 Dark achievements worth 1,000 gamerscore. It takes around 8-10 hours to unlock all of the achievements on Xbox 360.
Pursued Samantha.
Rescued Rose.
Talked to Samantha at Headquarters
Earned ten experience rewards for completing areas without raising the alarm.
Rescued Tom.
Learned all the powers and their upgrades.
Completed a level without raising an alarm
Reached the ruins of Sanctuary
Vampiric Powers used 50 times
Subdued 5 opponents and Fed on them
Made it through the club area at the Atlantis
Distributed 20 Power points
Visited the Geoforge Parks
Found 7 PDAs
Used Shadow Leap 50 times
Distributed 10 Power points
You met Blooming in the museum catacombs.
Killed 50 unaware opponents
Killed 50 opponents
Completed the Tutorial
You received your first blood – the first time is always something special …