Apart from pre-release events (which are designed to promote new products in a casually fun way rather than foster a healthy competitive environment), the oldest custom format was an experimental format known as Prop 15/3
The Ascended Heroes one is a newer favorite, and Azumarill from Stellar Crown is a really beautiful evolution card to pair with it
Some events, like Pokmons League Challenges, even let you earn points that could qualify you for bigger competitions like the World Championships
resolve one of the triggers of the other Shifter, unmorph it, copy the Seer, draw 2 (4 total)
Filling the battlefield with Food, Rat, and Army tokens allows you to utilize more permanents as resources
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