Here is the full list of all 39 The Banner Saga achievements. It takes around 25-30 hours to unlock all of the achievements on Windows.
Complete the game on Hard difficulty.
Complete the game on Hard difficulty level without losing a single battle.
Alette doesn't want to harm humans or varl...don't make her.
Get permission to destroy a varl landmark.
Whew...Egil made it the whole way!
Win a battle on Hard difficulty in the camp Training Tent using 6 rank 5 units.
Reach Boersgard in 120 days.
Acquire 5 rank 5 items.
Complete the game on Normal difficulty.
Complete the game without letting anyone in your caravan die to hunger.
Complete the game without ever getting low morale.
Fight 40 battles in a single playthrough.
Visit the godstone Stravhs.
Visit the godstone Bjorulf.
Visit the godstone Ingrid.
Visit the godstone Dundr.
Defeat an enemy with a Bowmaster.
Defeat an enemy with a Mender.
Defeat an enemy with a Warleader.
Visit the godstone Hadrborg.
Visit the godstone Marek.
Defeat an enemy with a Grudgewielder.
Visit the godstone Radormyr.
Defeat an enemy with a Backbiter.
Visit the godstone Denglr.
Defeat an enemy with a Provoker.
Defeat an enemy with a Siege Archer.
Defeat an enemy with a Warden.
Defeat an enemy with a Thrasher.
Visit the godstone Hridvaldyr.
Defeat an enemy with a Skystriker.
Defeat an enemy with a Hunter.
Defeat an enemy with a Strongarm.
Defeat an enemy with an Eagle Eye.
Defeat an enemy with a Shieldmaster.
Defeat an enemy with a Warmaster.
Defeat an enemy with a Spearmaster.
Defeat an enemy with a Raidmaster.
Defeat an enemy with a Warhawk.