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.
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 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 Warden.
Defeat an enemy with a Raidmaster.
Defeat an enemy with a Warhawk.
Defeat an enemy with a Backbiter.
Defeat an enemy with a Bowmaster.
Defeat an enemy with a Provoker.
Defeat an enemy with a Warleader.
Defeat an enemy with a Spearmaster.
Defeat an enemy with a Mender.
Visit the godstone Denglr.
Visit the godstone Hadrborg.
Visit the godstone Marek.
Visit the godstone Radormyr.
Visit the godstone Bjorulf.
Visit the godstone Dundr.
Visit the godstone Ingrid.
Visit the godstone Stravhs.
Complete the game on Normal difficulty.
Complete the game on Hard difficulty.
Complete the game on Hard difficulty level without losing a single battle.
Complete the game without letting anyone in your caravan die to hunger.
Complete the game without ever getting low morale.
Alette doesn't want to harm humans or varl...don't make her.
Reach Boersgard in 120 days.
Acquire 5 rank 5 items.
Get permission to destroy a varl landmark.
Fight 40 battles in a single playthrough.
Whew...Egil made it the whole way!
Defeat an enemy with a Grudgewielder.
Win a battle on Hard difficulty in the camp Training Tent using 6 rank 5 units.