You get it around level 25-50 if you are a death wizard. You can buy treasure cards of it at the bazaar but it is so costly it's really not worth it.
You can buy minions from Card packs in the Crowns Shop or you might come across some in the Bazaar.
there is the wooden one and troll and cyclops (all for myth only) fire elemental storm elemental and forest sprite a salamander like thing and something that is ice
lvl 28
at level 25/30