Diablo 4 characters who started on the Eternal Realm when it launched aren't supposed to be able to travel to the new, Seasonal Realm. But a newly discovered glitch that involves unplugging your internet has shattered the barrier between the two.
Seasonal characters in Diablo 4 carry over some of your progress from your Eternal Realm characters, but start at level 1 with no items or gold. Seasons are meant to be an opportunity to basically restart the trek to level 100 with the aid of a unique mechanic—in this case, it's the powerful Malignant Heart gems.
Apparently, the two realms aren't as separated—at least on a technical level—as Blizzard made it seem. Players have figured out that if you group up with a friend on the Seasonal Realm and then briefly disconnect from the internet, you can log back in as an Eternal Realm character with all of their stuff.
YouTuber Glitch Unlimited explains the forbidden realm-hopping technique and gives some examples of how it can benefit your characters on either side. Eternal Realm characters can sell their items for gold to use on the Seasonal Realm or put points in the Seasonal Blessing that increases XP gains and grind some levels out while they're on leave. They can also pick up and use any new Legendary items with seasonal aspects on them. Malignant Hearts, however, won't work outside the Seasonal Realm.
Your Eternal Character can also leave powerful items for your Seasonal Character by dropping them in the fourth stash tab, which, for whatever reason, is shared between the two. That means you can gift your Seasonal character a few powerful Legendary or Unique items if you've been unlucky so far.
Given the strict rules for what transfers between both Diablo 4 realms, I'm sure this glitch won't last long before Blizzard fixes it. If it didn't so obviously break the rules by tricking the server to seemingly misunderstand where to put your characters, I'd say make use of it while you can. But it's very possible Blizzard will punish anyone for using this, so I'd personally stay clear of it.
There's enough changing in patch 1.1.1 that it might make the items you transfer over only useful for about a week. The full patch notes will go live tomorrow, and the patch will drop next week on August 8.