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Diablo 4 will have Seasons and Blizzard has stated that they plan to have up to four Seasons per year[1].

Many of you enjoy the Seasons in Diablo II: Resurrected and Diablo III and have asked for more extensive season support. We agree one of the most fun ways to play Diablo is through Seasons, so we're making the first one available soon after launch and building a dedicated team to bring you up to four Seasons a year, each with major new features, questlines, enemies, legendary items, and more.

Characters

Like Diablo 3, Diablo 4 characters from the previous season are moved to the "Eternal Realm" where players can continue to play their charcaters.

A new character will have to be created at the start of each Season.

To play in the new season, you’ll create a fresh character and experience the new seasonal features and content while leveling up alongside other players. This, along with capping paragon points in Diablo IV, ensures that your effort and skill—measured by both dexterity and theorycrafting —determine how powerful your character becomes. It also allows players who missed the last season to participate. This season design requires that all sources of character power come from playing the game, so you will not be able to pay for power in Diablo IV.

Season Pass

Diablo 4 will introduce the Season Pass. The Season Pass will include Free Tiers and Premium Tiers. Premium Tiers will provide no in-game power advantage over the Free Tiers. Premium Tiers are focused on cosmetics and "Premium Currency". Blizzard outlines this as follows:

The Season Pass has Free Tiers and Premium Tier

 Throughout the pass players can earn a variety of rewards for free, just by playing. At any point during the Season, players can purchase the Premium Pass to unlock the ability to earn Premium rewards tiers containing seasonally themed Cosmetics and Premium Currency.

The Season Pass awards cosmetics

Like the Shop, these don’t affect gameplay. Additionally, certain cosmetic types are exclusive to the Season Pass.

The Season Pass awards Premium Currency

Players can spend this currency on cosmetics offered in the shop.

The Season Pass awards free Season Boosts

Boosts accelerate players’ progress for the duration of the Season. For example, a Season Boost might accelerate XP earned to make leveling multiple characters within a season faster. Because they affect gameplay, Season Boosts are free rewards for all players. We want to be clear that players can’t unlock Season Boosts more quickly through purchases--there is no way to unlock more boosts, or boosts at a faster pace, by spending money.

Players can purchase Tiers-but they won’t speed up getting Season Boosts

Players can’t upgrade Season Boosts just by purchasing Tiers, because they’ll also have to earn level milestones to apply them. All other Tier rewards can be unlocked instantly by purchasing Tiers. In other words, there’s no way to shortcut getting Season Boosts by buying Tiers; they must be earned.

The Season Journey accelerates Season Pass progression

While any play style can progress through the Season Pass, min-maxers can focus on Season Journey objectives to advance more quickly.