Relentless Dead
Creature — Zombie
Menace
When this creature dies, you may pay . If you do, return it to its owner's hand.
When this creature dies, you may pay 
. If you do, return another target Zombie creature card with mana value X from your graveyard to the battlefield.
Ryan Yee
Shadows over Innistrad (SOI)
• #131Rulings
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.
(wotc • 2016-04-08)
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.
(wotc • 2016-04-08)
Rulings
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.
(wotc • 2016-04-08)
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.
(wotc • 2016-04-08)