It has been confirmed by Blizzard that Diablo Immortal will not be playable when you’re offline.
In an interview at BlizzCon, Wyatt Cheng, lead game designer on Diablo Immortal, confirmed that you can't play the game if you’re offline, stating that "the game would not allow you to." You are able to play solo, to an extent (in the demo shown at BlizzCon, the final dungeon could be completed solo or in a party, with the game allowing you to choose), but the game will not let you play if you don't have an internet connection.
It’s not entirely surprising, given how many mobile games do require a constant internet connection. It does mean that playing Diablo Immortal on a morning commute may be a little challenging for some, though.
Part of the game is developed around being massively multiplayer, meaning the multiplayer element is crucial. When roaming an overworld, you might come across other players also running about and slaying demons if they're in the same area as you, and dungeons can be entered in parties. There's also social elements, such as Battle.net integration, which add to the list of reasons why Diablo Immortal is online-only.
Those at BlizzCon have had a chance to play it and, in contrast to the reaction online, it’s a pretty solid action RPG. It’s just a shame that the world hasn’t been able to see the demo too.