How do you get the 2018 LMS G-Rex?
It was sold in the League of Legends store. Riot's description in the client reads: “This icon was released in the store for purchase during the 2018 esports season.”
Is the 2018 LMS G-Rex still available?
Probably not. Riot tied the 2018 LMS G-Rex to 2018 (store purchase), and it does not normally bring back past event rewards or bundles.
