How do you get the FPX - From Fire Reborn Icon (Premium)?
It was a mission or event reward. Riot's description in the client reads: “This icon is to commemorate your team FPX completing the team mission of MAKE/BREAK challenges.”
Is the FPX - From Fire Reborn Icon (Premium) still available?
No. Riot tied the FPX - From Fire Reborn Icon (Premium) to 2021 (mission reward), and it does not normally bring back past event rewards or bundles.
