Evolving a hero unlocks extra passive and active skills. One star abilities can be unlocked by leveling from 1-10, but you must evolve to unlock two star and level 11-20 abilities. Before you begin to evolve heros, you must put 30 skill points into the tier 1 skill tree to unlock the two star class evolution skill node. This allows you to evolve a hero of that class only; you must earn enough skill points to unlock another two star class evolution skill node. Thus, it is recommended that you are sure you want to play a particular hero before investing your points into unlocking their evolution. You need to invest skill points in a class skill line before you can unlock its evolution. After unlocking the two star class evolution, you also must meet the following requirements:

Hero level: You need to invest 10 levels into the hero that you want to level up. Each level will progressively take more XP to level up.

Hero XP: In addition to the XP you need to level from 1 to 10, you need an additional amount of XP to evolve a hero. Retiring a hero you no longer want to use will return a large amount of invested XP to you -- so if you are short on XP, consider retiring a hero you no longer wish to use.

Evolution crafting material: Each star tier requires a different crafting material. Two star evolutions require Pure Drops Of Rain, three star evolutions require Lightning In A Bottle, etc. Evolution crafting materials can be earned by completing quests, missions, retiring a hero, and inside of certain loot llamas.

Training manual: Can be acquired by completing quests, as mission rewards, or by retiring survivors and heroes.

The hero evolution screen displays all of the requirements, as well as any stats you will gain. After selecting evolve, your hero's star level will increase and you can begin working towards the next level of evolution.