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MH3: Chthonian Nightmare, Enchantment 1B When Chthonian Nightmare enters the battlefield, you get {EEE] (three energy counters). Pay X {E}, Sacrifice a ...
When Chthonian Nightmare enters the battlefield, you get {EEE] (three energy counters). Pay X {E}, Sacrifice a creature, Return Chthonian Nightmare to ...
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So whenever you kill the Guide or Pride, they cast Nightmare to get one back which creates another cat token so they can repeat the process when needed. Pretty annoying -- at least to anyone not playing UW Control and most people have a sideboard dedicated mostly to handling that, these days, since you want to punch Gandalf in the face or lose quick enough that it doesn't hurt.
I think the combo I faced mostly annoyed me because neither Guide nor Pride is a 1-drop they'd ever really make for standard so it's basically "let's print an entire deck for old formats." They're not rescuing energy, really, it's basically just creating something for people with $$$ to try out and throw against the wall, maybe it works, maybe it doesn't, but there's no way to have any emotional connection to the cards in play since they were basically constructed to only be in one deck and only in the current moment.