After much speculation some light has finally been shed on the details of the first season in Diablo IV. Like its predecessor, Diablo IV is designed to be a living game that evolves and grows in the coming years. The long-term content schedule includes seasons which kick off on July 20 with Season of the Malignant. This Malignant event will include a new questline with some new faces and enemies, new powers, the first Season Journey and the first Battle Pass.
As is tradition, seasons will give all players on opportunity to start fresh with new features and gameplay mechanics. With everyone starting on equal footing with a new character, players are free to try out new classes and builds that they’ve been curious about or stick with their familiar favorites. Leveling up a new character doesn’t mean it completely starts from scratch as it brings the previous renown gained from discovering areas in the map and finding Altars of Lilith. If the mount is unlocked it’ll be available to use immediately with the new character. As the season ends the seasonal character will migrate to the Eternal Realm where the player will have permanent access to it, contrary to the rumor and fear the characters will be deleted at the end of each season.

Listed below are some of the details of the season, taken directly from the press release:
A New Adventure Arrives:A dangerous new threat roams the land in the form of Malignant Monsters. Team up with Cormond, a former priest, to stop the spread of the Malignant at all costs.
Fight the Infestation:Destroy these Malignant Monsters by drawing the corruption from them and trapping their Malignant Hearts, which can then be inserted into Rings and Amulets.
No Prior Experience Required:New players will be able to jump into seasonal realms and play through the main campaign as they learn the basics and then go straight into the season content and new systems introduced. They will also be able to progress along parts of the season journey and battle pass as they play. Hardcore players will be able to create new characters and experiment with new builds or classes as they play through the season journey. Seasons add an entirely new way to customize characters and build up their power as they take on end game challenges.
Season Journey:This multi-chapter objective system rewards players for completing each chapter. With multiple Chapter, you can earn Favor from rewards which will progress the Battle Pass.
Battle Pass:By earning Favor through regular gameplay or purchasing optional tier skips, players can earn rewards as they complete Battle Pass tiers. The Battle Pass offers a free tier where everyone can earn Season Boosts to accelerate player progress. If a player chooses to purchase the paid tier, they can also earn exclusive Battle Pass cosmetics.