With enough bodies on the board to benefit, you can really speed up the clock on your opponent
Ragavan allows midrange decks to take an aggressive stance, which has been tough for a number of years
As such, you can think of flash as instant, but for your permanents instead
As for the card, it's a copy of Akroma's Will, an instant that lets you choose between all your creatures gaining flying, vigilance, and double strike, or lifelink, indestructible, and protection from each color until the end of the turn
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Even after multiple reprints, youre still looking at $15 for this banger on the low end, or $35 if you want this fancy Source Material version