Note: This is a missable achievement, since you can mix destroying and keeping the things, annulling the
Catharsis and
Redemption achievement!
After progressing through the story and obtaining the ability to manipulate fire, you will come back to the tree house. There, Finn will wait for you and kind of "explode", setting four piles of stuff on fire.
Simply go outside, draw fire (
) from the brasier then add it (
) to a burning pile. You have to add fire to every burning fire to get this achievement.
Look out for the other achievement!After obtaining this achievement, the game won't autosave until after you speak to Finn again, so just quit the game and reload it by selecting "Continue" in the main menu. You'll start again at the bottom of the ladder leading to the tree house, allowing you to go for the Redemption achievement without starting a whole new playthrough!