How do you get the Full Moon Poro Icon?
It was a mission or event reward. Riot's description in the client reads: “Make a wish to the full moon! This icon was granted through the 2020 Autumn Holiday event.”
Is the Full Moon Poro Icon still available?
No. Riot tied the Full Moon Poro Icon to 2020 (mission reward), and it does not normally bring back past event rewards or bundles.
