OBB (stands for "Oriented-Block-based" and can be also referred to as a compressed texture pack) files are automatically downloaded when you set up the modded apk on your android device, they contain all graphic elements of the game such as 3D models, backgrounds, quests descriptions etc., allowing gamers to play Unkilled Mod APK in offline mode with no internet connection required at all! And since Unkilled supports more than one map - we've had to use this way of making sure that gamers will not get stuck at the load screen when playing offline!