Your heroes gain Experience Points which are used for hero leveling.
You can receive XP for passing through Campaign missions (only those heroes who are in the party gain XP). Besides you can level up your heroes using the EXP potions, which are given as a reward for mission completions and raids, can be found in caskets and purchased in the ingame shops.
In the hero leveling interface you can:
- Use a single potion (by selecting required potion and clicking on hero portrait)
- Increase hero level by clicking "+" near their level. In this case you will spend potions starting from selected ones to the smaller ones if you have them in stock.
NOTE: The maximum level of a hero depends on the team level.