When you set this scheme in motion, you may pay {X}. If you do, put each nonland permanent target player controls with mana value X or less on the bottom of its owner's library.
Every Last Vestige Shall Rot is not legal in any format.
Rulings
2010-06-15
You choose the target player when the ability triggers. As the ability resolves, you choose a value for X and decide whether to pay {X}. If you do decide to pay {X}, it's too late for any player to respond since the ability is already in the midst of resolving.