Diablo III does not require a monthly subscription. Once you have bought a copy of the game it is yours to play, however it does require a free Battle.net account to play. In addition, the game does use micro-transactions, allowing you to purchase in-game items with real money, but this is optional.
Diablo III is a dark fantasy role-playing game developed by Blizzard Entertainment and is the third game in the popular series. It was released in North America on May 15, 2012 for Windows PC and Mac OS X. Early reviews for the game have been favorable and it has received a metascore of 86% from game critics.