Butcher Knife of Paralysis

{t}: Tap target creature. That creature doesn't
untap during its controller's untap phase as long
as Butcher Knife of Paralysis remains tapped.
Whenever a creature tapped by Butcher Knife of
Paralysis becomes untapped, it deals damage to
you equal to its power.
You may choose not to untap Butcher Knife of
Paralysis during your untap phase.