How do you get the Harrowing 2013 - Vilemaw?
It was a mission or event reward. Riot's description in the client reads: “Earned by playing one game in any map during Harrowing 2013.”
Is the Harrowing 2013 - Vilemaw still available?
No. Riot tied the Harrowing 2013 - Vilemaw to 2013 (mission reward), and it does not normally bring back past event rewards or bundles.
