As an additional cost to cast this spell, exile a creature card from your graveyard.
Undying (When this creature dies, if it had no +1/+1 counters on it, return it to the battlefield under its owner's control with a +1/+1 counter on it.)
1/22/2011 When Relentless Skaabs returns to the battlefield because of its undying ability, it’s not being cast. You won’t exile a creature card from your graveyard.
4/15/2013 You must exile exactly one creature card from your graveyard to cast this spell; you cannot cast it without exiling a creature card, and you cannot exile additional creature cards.
4/15/2013 Players can only respond once this spell has been cast and all its costs have been paid. No one can try to otherwise remove the creature card you exiled in order to prevent you from casting this spell.