Diablo 4 seasons will add "fresh concepts and ideas" four times a year

gamesradar.com:

Diablo 4 will begin its seasonal content in July and introduce "fresh concepts and ideas" four times a year going forward, Blizzard said in a comprehensive breakdown published Wednesday.

In a new developer livestream and accompanying blog post (opens in new tab), the Diablo 4 team gave a detailed overview of the long-awaited sequel's post-launch plans, including its seasons, free and paid battle passes, the in-game shop, and more. Let's dig into it.

There will be four Diablo 4 seasons per year, with the first one beginning sometime in mid to late July. Each season will have its own unique theme, and while Blizzard didn't get too much into specifics, it did confirm the first season won't be zombie-themed as some have speculated. The theme of the first season will be discussed in detail at some point after launch. Each season will have its own self-contained questline that'll guide players through newly introduced mechanics and introduce them to new and old characters alike, but it'll only become available once you've completed the main campaign. There will also be new unique items and powers added every season, and once that season is over they'll be added to the "Eternal Realm," which is just Blizzard lingo for the normal base game.

Within Diablo 4's seasonal structure is something called Season Journey, which Blizzard likens to "the North Star" in the night's sky. Basically, it'll tell you what you need to do to quickly level-up your battle pass, which will involve completing objectives that make up chapters. 

Finishing a chapter will earn you seasonal rewards like crafting materials and Legendary Aspects for your Codex of Power. Completing objectives will also reward you with Favor, the in-game resource - which can