How do you get the Eternum Cassiopeia Icon?
It was a mission or event reward. Riot's description in the client reads: “Earned by Mystery Icon of Midseason Essence Emporium.”
Is the Eternum Cassiopeia Icon still available?
No. Riot tied the Eternum Cassiopeia Icon to 2017 (mission reward), and it does not normally bring back past event rewards or bundles.
