Chaos is similar to a morality system found in other games. Actions such as murder, alerting guards, having bodies found, and all other disruptive (non-stealthy) actions will increase your “Chaos Level”. Higher chaos level leads to a more chaotic world environment; filled with creepers (rats), weepers, guards, and a more gloomy Dunwall in general.
Your chaos level is cumulative across all of the missions. It will go up and down based on how you approach an entire mission, in regards to the main target and optional objectives. If you choose to outright assassinate your main target you will greatly increase your total chaos level. Conversely, if you opt for the non-lethal alternative you will drastically lower your chaos level for that mission.
In order for the trophy to unlock you must finish the final mission with a total chaos level for the entire game that is “high”. It doesn’t matter if several missions are marked as “Low Chaos” in the summary, as long as by the time you reach the final mission you have pushed it back into the “High Chaos” range. It is ideal to this trophy in tandem with Mostly Flesh and Steel, because you will not have to rely on multiple powers to stealthily make your way through the game.