You can't play Diablo Immortal offline

While Diablo III can be played offline in all platforms, including the recently released Switch version, the same will not be true to Diablo Immortal. The iOS and Android game will require an online connection at all times, as confirmed by Lead Game Designer, Wyatt Cheng, during BlizzCon.

While you can't play offline, you will be able to play a large portion of the content solo, even if Diablo Immortal is even more focused on social elements and co-op play than previous Diablo games.

Another thing about Diablo Immortal for Diablo fans to complain about we suppose, then again, few online focused mobile games allowed offline play (as you need a connection to the server to register progress, currencies, etc).

Thanks, VG247.