How do you get the Master's Challenge Season 11 Icon?
It was a ranked reward. Riot's description in the client reads: “This icon was earned by making it to the top 2000 in China's Master's Challenge server.”
Is the Master's Challenge Season 11 Icon still available?
No. The Master's Challenge Season 11 Icon was a ranked reward. To earn it, you had to reach Master during Season 1 (2000). Riot does not hand out past ranked rewards again, so you cannot earn this icon anymore.
Master's Challenge Season 11 Icon is one of the rarest LoL summoner icons.
