All heroes are ranked in one of three rarity tiers. These tiers are, from low to high, Common, Legendary+ and Ascended. Generally speaking, a hero with a higher rarity is stronger than one with a lower rarity. Below, you will find more about the three tiers, where to find them and how to use them.
The first tier is the Common tier. Common heroes can be received from hero pulls in The Noble Tavern. These heroes are, for almost all players, completely expendable. The newest players will likely use their free Ulric in their first stages, but after that, Common heroes aren't useful at all. Luckily, they can be retired at the Rickety Cart. This will reward the player with a small amount of Hero's Essence and some Hero Coins, which can be traded in for Soulstones for heroes of higher rarity tiers.
Like the Common heroes, these can be received from pulls in The Noble Tavern. When pulled, they will usually be of the rare Ascension tier, but they will occasionally show up as Elite heroes. Furthermore, they can be bought or found in the form of Rare or Elite Soulstones.
Legendary+ tier heroes are quite useful in the early to mid-game. Unfortunately, they can not ascend past the Legendary+ ascension tier. Also, their maximum level is capped at 160. As the player passes that level, their usefulness quickly decreases. In the late game, most Legendary+ tier heroes are only useful as fodder for ascending Ascended tier heroes. An exception is Arden, whose powerful crowd control abilities make him a viable option even when outleveled.
Now we're talking. Ascended tier heroes are the most powerful and a good late-game setup will only consist of these heroes. As the name suggests, these heroes can be brought all the way to the Ascended ascension tier. Like all heroes, Ascended tier heroes can be received from The Noble Tavern. However, they are quite rare, and even a 10x pull often won't contain an Ascended tier hero. Luckily, there are more ways to get them. Elite Soulstones often contain Ascended rarity heroes. Furthermore, when enough pulls have been done in the Tavern, one can pick a free Elite hero of a chosen Faction. This can often be an Ascended rarity hero. Finally, events often let you buy specific heroes for event tokens. These heroes will usually also be of Ascended rarity. When received, Ascended rarity heroes will always be of the Elite ascension tier.