The dark fantasy esport Dota 2 is undeniably Steam’s killer app, according to the number-crunching-smiths at Ars Technica, who found that over 25 million copies have been downloaded and played on Valve’s popular digital download service. We shouldn’t be shocked that such a hardcore, complex game—even one that takes months of practice to achieve anything close to competence—would be the game that everyone owns and plays, considering that Dota 2 is a worldwide phenomena, and that Steam is the only way to play it.  Ars’ numbers are dominated by hardcore, competitive games. With the exception of Portal, the top ten…