Giant Oyster (TSB-022) - Time Spiral Timeshifted

You may choose not to untap this creature during your untap step.
: For as long as this creature remains tapped, target tapped creature doesn't untap during its controller's untap step, and at the beginning of each of your draw steps, put a -1/-1 counter on that creature. When this creature leaves the battlefield or becomes untapped, remove all -1/-1 counters from the creature.