Sphinx of Resonance

Flying, Prowess
{1}{u}: Monstrosity 1
When an opponent casts a spell with converted mana cost equal to the number of +1/+1 counters on Sphinx of Resonance, add a +1/+1 counter to him.
...though a sound be very slight in the open air, it has a loud and alarming resonance to creatures
-Aristotle, The History of Animals