Nissa's ShepherdEmeria Shepherd Buy + Nissa, Genesis Mage Buy + Doubling Season Buy
I found this as a weird engine that requires some pretty hefty fuel to continue, but makes a very neat interaction. It isn't infinite by itself, but you can more or less repeat it every plains you get for free and it costs 7 mana to repeat. It kind of looks like this:
: Ult Nissa, if a plains hits, repeat this process, if a non plains land hits, return Nissa to hand.
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