Yesterday, the NPD Group reported that Diablo III was the top-selling game of May, but just how well did it perform? In a note to investors today citing NPD data, research firm Cowen & Co. revealed that Blizzard's new dungeon crawler moved 1 million units at retail during the month. Blizzard reported last month that the game sold 3.5 million units across physical and digital retailers during its first 24 hours. A million marched into the mouth of hell in May.

　　In addition to sharing sales figures for Diablo III, the firm revealed that Rockstar Games' Max Payne 3 and Ubisoft's Ghost Recon: Future Soldier each sold just north of 400,000 during May. Both titles "underperformed," according to the firm. This time last year, Rockstar's L.A. Noire was able to move nearly 900,000 units.

　　Overall industry sales for May slid 28 percent year-over-year, and Cowen & Co. does not foresee a turnaround anytime soon. The firm projects June and July software sales will tank 30 percent year-on-year, due to "continued Wii and handheld weakness" and a dearth of new Xbox 360 and PlayStation 3 titles.

　　Looking even further into the future, the company said the release slate for the 2012 holiday period "looks a bit lighter on the core side than last year." This results in the firm projecting the downturn to continue, despite the launch of Nintendo's Wii U.