Hek, Master Slaver

Swift (This creature cannot be blocked except by creatures with flying or first strike.)
{2}{p/r}: Put a +1/-1 counter on target creature you control.
{2}{p/r}: Put a -1/+1 counter on target creature you control.
Whenever a creature dies due to Hek, return a creature card with CMC X or less to the battlefield under your control, where X is the number of counters that were on that creature.