Relentless Dead

Oracle Text

Menace (This creature can't be blocked except by two or more creatures.)

When Relentless Dead dies, you may pay B. If you do, return it to its owner's hand.

When Relentless Dead dies, you may pay X. If you do, return another target Zombie creature card with converted mana cost X from your graveyard to the battlefield.

Card Rulings

4/8/2016 Relentless Dead’s last two abilities trigger at the same time and can be put on the stack in either order. The ability put on the stack first will be the last to resolve.
4/8/2016 You decide whether to pay for each triggered ability as it resolves. If you do, the rest of that ability’s effect happens before any player may take actions.