Hello, the Obsidian summoner belongs to the Chaos Legion set, so you cannot use it in the Modern format. That summoner can be used in tournaments, brawls, Wild format, and Survival.
Currently, in the Modern format, you play with the Foundation set and the two current sets, Rebellion and Arcane Conclave. If you want to buy summoners, I suggest the Arcane Conclave set, which will remain active in the Modern format for longer.
You can also get summoners and units from the Foundation set by playing the campaign mode and winning them in the Frontier format.

!BBH