Heroes are the means by which a player defeats monsters in the dungeon and gains Experience (XP) and Victory Points (VP). The basic game of Thunderstone involves a random selection of four sets of heroes as well as the Militia that are included in every session.
Each hero, and it's various versions, has a default strength rating listed in the purple "shield" icon located above the level identifier.
This value is used to determine what weapons the hero may equip as well as in various Dungeon/Battle events such as when fighting the Revenant.
Each hero has a total of three "levels", with them gaining higher combat ability, strength for equipping weapons, and extra effects to use when visiting the Dungeon or Village.
The final level of a hero also provides VP for scoring purposes.
To level a hero up, a player decides to visit the Village, and after making any purchases and resolving any Village effects, they may spend the amount of XP shown on the hero card's bottom left in red, destroying the hero and then taking the next level version from the Village and adding it to their discard pile for later use. As the hero is now in the discard pile, multiple level ups on a single hero are not possible in a single visit to the Village.
If there are no more copies of the next level, that hero cannot be levelled up anymore. Likewise, players may not skip a level by spending the total XP cost to reach the last level. They MUST level up in order (1>2>3).
Higher level versions may be purchased for gold only when they are on top of the pile, meaning all level 1 versions must have been bought first.
Some players use a variant rule in which the hero is not destroyed when levelling, but is placed back into its respective place in the Village. This allows players to obtain heroes if they fall behind on gold, as well as delaying the ability to simply purchase higher level heroes.