“Diablo 4 includes Seasonal Content that runs for certain duration, and requires you to create new Characters made exclusively to experience these new adventures and changes on a specific realm.
“This page includes information on how to create a Character on the Seasonal Realm, and how to prepare for the Seasonal Content.
“Note that any characters created before the new Season are bound to the “Eternal Realm” and cannot participate. At the end of each Season, any characters created for the Seasonal Realm will be migrated to the Eternal Realm as well.
“Before you create your new Character, you’ll want to make sure your new Season 1 Character will enjoy all the benefits unlocked by your main Character on the Eternal Realm. After Patch 1.1.0 the unlockable rewards that carry over to other characters has been updated to include map reveals and Altar of Liliths.”
Read more at IGN