Koskun Falls is basically very similar to Propaganda in Blue and Windborn Muse in White. The Blue Enchantment and White creature both offer the ability to protect you from opponent’s attacks by forcing your opponent to pay 2 colorless mana for each creature he or she attacks you with. Koskun Falls does just that, but for 4 mana instead of 3 and with an upkeep cost of tapping down one of your creatures.
Are there any decks that actually play this? In Commander, Daxos the Returned decks have taken advantage of its ability, especially since that deck has no trouble producing a token to tap down on a regular basis. It’s not an ability you usually expect from black, and while the upkeep cost can be annoying, it’s actually a pretty useful card in a deck that doesn’t ordinarily play Windborn Muse and can’t play Propaganda.
After spiking to $5, Koskun Falls came back down to earth around $3 or so. As a reserved list card, there aren’t any more of these being printed, so you can feel safe picking up any reasonably priced copies that you find.