How do you get the Harrowing 2014 - Mark of the Betrayer?
It was a mission or event reward. Riot's description in the client reads: “Earned by winning three games of Hexakill during the 2014 Harrowing event.”
Is the Harrowing 2014 - Mark of the Betrayer still available?
No. Riot tied the Harrowing 2014 - Mark of the Betrayer to 2014 (mission reward), and it does not normally bring back past event rewards or bundles.
