Before you can install the game, you need to allow your phone to install apps from sources other than the Google Play Store.
Since these are unofficial fan projects, you won't find them on the Play Store. sonic.exe spirits of hell round 2 android port
However, if you are a casual Sonic fan expecting a platformer, or if you have a low tolerance for trial-and-error gameplay (you will die. A lot. Instantly.), this is not for you. The difficulty is archaic, the jumpscares are loud, and the psychological trickery (fake file deletes) might be too stressful for a mobile gaming session on the bus. Before you can install the game, you need