If the internet goes out mid-game, you get a 60-second countdown timer. If connection isn't restored, the game boots you to the title screen, erasing all unsaved progress (yes, no local save files).
While you cannot play offline, there are settings that mimic certain aspects of an offline experience: diablo iv offline mode install
If you need a true offline experience, consider these alternatives: If the internet goes out mid-game, you get
The game features a rotating cosmetic shop and seasonal Battle Passes that rely on a constant connection to track purchases and progress. Can You Bypass the Online Requirement? If the internet goes out mid-game
By hosting game logic on their servers, Blizzard can prevent common hacks, item duplication, and modding that often plague offline games with local save files.