How do you get the Old God Nocturne Icon?
It came in a bundle. Riot's description in the client reads: “This icon was acquired by purchasing one of the Elderwood bundles in 2019.”
Is the Old God Nocturne Icon still available?
Probably not. Riot tied the Old God Nocturne Icon to 2019 (bundle), and it does not normally bring back past event rewards or bundles.
