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.
Complete the game without ever getting low morale.
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.
Fight 40 battles in a single playthrough.
Complete the game on Normal difficulty.
Complete the game without letting anyone in your caravan die to hunger.
Visit the godstone Stravhs.
Reach Boersgard in 120 days.
Visit the godstone Bjorulf.
Defeat an enemy with a Bowmaster.
Visit the godstone Ingrid.
Visit the godstone Dundr.
Defeat an enemy with a Mender.
Defeat an enemy with a Grudgewielder.
Acquire 5 rank 5 items.
Defeat an enemy with a Backbiter.
Visit the godstone Hadrborg.
Defeat an enemy with a Warleader.
Visit the godstone Marek.
Visit the godstone Radormyr.
Visit the godstone Denglr.
Defeat an enemy with a Provoker.
Defeat an enemy with a Siege Archer.
Defeat an enemy with a Thrasher.
Visit the godstone Hridvaldyr.
Defeat an enemy with a Skystriker.
Defeat an enemy with a Strongarm.
Defeat an enemy with a Hunter.
Defeat an enemy with an Eagle Eye.
Defeat an enemy with a Shieldmaster.
Defeat an enemy with a Spearmaster.
Defeat an enemy with a Warmaster.
Defeat an enemy with a Warden.
Defeat an enemy with a Raidmaster.
Defeat an enemy with a Warhawk.