Titan Boa

You may choose not to untap Titan Boa during
your untap step.
{t}:For as long as Titan Boa remains tapped,
target tapped creature an opponent controls
doesn't untap during its controller's untap step,
and at the beginning of your upkeep, put a -1/-1 counter on that creature. When Titan Boa leaves
the battlefield or becomes untapped, remove two
-1/-1 counters from the creature.