Here is the full list of all 16 Banners of Ruin achievements.
Forfeit a character card choice
Win a combat with a combined Party Vitality of 3 or less
Defeat the Garrison Commander
Play 20 cards in a single turn
Have 30 or more Charge during combat
Apply 30 Poison in a single combat
Banish 10 cards in a single combat
Reach 10 Stamina during combat
Reach a party of 6 characters
Reach 40 or more cards in your deck
Win 50 combats
Defeat the Elite Brutes
Hire a character at the tavern
Defeat the Elite Bruisers
Complete the Tutorial
Defeat 3 enemies in a single turn