Monastery Mentor proved itself to be one of the best finishers in older formats like Vintage, where it's easy to chain together cheap spells and cantrips to potentially make a lethal army of prowess tokens in just a single turn, which is certainly strong but still doesn't really feel broken to me
There were some early sets that were enormous in size like Legends , but other than the erroneous Arabian Nights Mountain, basic lands and sealed-deck staples weren't an auto-include part of sets
The shape works well because it is easy to shuffle, comfortable to hold, and practical for game play
All standard-sized card sleeves will fit
In case you've heard the term but don't know the cards, here's a rundown of the original Power Nine: Honorable Mention: Library of Alexandria, arguably Magic's most powerful land, rounds out most people's list of the Power Ten