How do you get the Bilgewater Twisted Fate Act 1?
It was a mission or event reward. Riot's description in the client reads: “Earned by participating in the 2015 Twisted Fates' Act 1 Bilgewater Faction event.”
Is the Bilgewater Twisted Fate Act 1 still available?
No. Riot tied the Bilgewater Twisted Fate Act 1 to 2015 (mission reward), and it does not normally bring back past event rewards or bundles.
