In 12 days, your Diablo 4 season 1 characters will be retired and sent back to the Eternal Realm

pcgamer.com:

Diablo 4 guide: Everything you need
Diablo 4 mount: Race across the map
Diablo 4 waypoints: How to fast travel
Diablo 4 level unlocks: New vendors

When Diablo 4 season 2, or the Season of Blood, begins on October 17, everyone playing a seasonal character will have to start over from scratch.

Season 1 characters will be kicked into the Eternal Realm, where non-seasonal characters live, with all their gold, items, and crafting materials. Any items that were in that character's stash will transfer into a new withdraw-only chest that they can access from the Eternal Realm. The withdraw-only stash will hold your items until the end of season 2—which should be roughly three months long—before they're permanently deleted. And any Malignant Hearts socketed into your gear or sitting in your bags will disappear in the process.

Some things will technically stay, though. Blizzard just announced that any rewards from gaining renown will now be permanent across all your current and future characters. You won't have to hunt down Altars of Lilith or clear out sidequests over and over again anymore. And if you've at least completed the prologue quest, you can now skip the rest of the campaign on new characters.

Unlike Diablo 2 Resurrected, you won't be able to move your character (in name and look only) back to the Seasonal Realm. They're forced into retirement after one job and can't return.

All the quality of life and class balance changes coming with season 2, however, will affect the Eternal Realm if you can't bring yourself to abandon your characters. But only fresh season 2 characters will be able to wield the new Vampiric Powers.

Here's a breakdown of everything that carries over from your Seasonal Realm characters to the Eternal