Unexpectedly Absent
Oracle Text
Put target nonland permanent into its owner's library just beneath the top X cards of that library.
Card Rulings
6/8/2016 If there are fewer than X cards in that player’s library, put that permanent on the bottom of that library.
6/8/2016 If you choose 0 as the value for X, put that permanent on top of its owner’s library.
Putting something on the top of your opponent's deck at instant speed is pretty powerful at just 2 mana. Sure, it's not the greatest removal spell ever, and it was overhyped before its first release in C14, but it's still rock solid.