How do you get the Season 2 Reward - Diamond 5v5?
It was a ranked reward. Riot's description in the client reads: “Earned by achieving Diamond in the 2012 ranked season in 5v5.”
Is the Season 2 Reward - Diamond 5v5 still available?
No. The Season 2 Reward - Diamond 5v5 was a ranked reward: to get it you had to reach Diamond in Ranked 5s during Season 2 (2012). Riot does not give out past ranked rewards again, so it cannot be earned anymore.
