Every Last Vestige Shall Rot #11★
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When you set this scheme in motion, you may pay 
. If you do, put each nonland permanent target player controls with mana value X or less on the bottom of its owner's library.
“"May bloodberries grow from your remains."”
Illustrated by Martina Pilcerova
Rulings (1)
Jun 15, 2010
You choose the target player when the ability triggers. As the ability resolves, you choose a value for X and decide whether to pay . If you do decide to pay , it's too late for any player to respond since the ability is already in the midst of resolving.