Michiko, Heartbeat Stealer

{t}: Choose up to two target creatures you don't control. Put a mark counter on each of those creatures. Michiko, Heartbeat Stealer doesn't
untap during your untap step as long as a
creature chosen this way has a mark counter on
it. Activate this ability only during your turn.
Whenever one or more creatures attack, you may
untap Michiko. If you do, destroy each attacking creature with a mark counter on it, then remove
all mark counters from each creature.