Rotted Mana-Beast

If unused mana would be emptied from a mana pool, put a +1/+1 counter onto Rotted Mana-Beast for each mana emptied this way.
{t}, remove any number of +1/+1 counters from Rotted Mana-Beast: Target creature gets -1/-1 until end of turn for each +1/+1 counter removed this way.