How do you get the The Big One Icon?
It was a mission or event reward. Riot's description in the client reads: “Earned by completing A Guardian's Pinnacle mission in Invasion mode on hard as a 5-person premade with an S or higher.”
Is the The Big One Icon still available?
No. Riot tied the The Big One Icon to 2017 (mission reward), and it does not normally bring back past event rewards or bundles.
