Is Relentless Dead legal in Standard, Modern, and Commander?

Relentless Dead is legal in: Modern, Pioneer, Legacy, Vintage, Commander, Historic.

What type of card is Relentless Dead?

Relentless Dead is a Creature — Zombie. Its oracle text reads: "Menace (This creature can't be blocked except by two or more creatures.) When this creature dies, you may pay {B}. If you do, return it to its owner's hand. When this creature dies, you may pay {X}. If you do, return another target Zombie creature card with mana value X from your graveyard to the battlefield."

Relentless Dead

Creature — Zombie

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

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.