Ningo, Soul Warper

At the beginning of each end step, gain a puppet counter.
{t}: Look at target opponent's graveyard and
choose any number of creatures with mana value
X or less, where X is the number of puppet
counters you own. Place those creatures into the
battlefield under your control with a string
counter. At the end of your turn, for each
creature with a string counter, pay {1} or sacrifice
that creature.