Is Spawn of Thraxes legal in Standard, Modern, and Commander?

Spawn of Thraxes is legal in: Modern, Pioneer, Legacy, Vintage, Commander.

Spawn of Thraxes

Q: What type of card is Spawn of Thraxes?

Spawn of Thraxes is a Creature — Dragon. Its oracle text reads: "Flying When this creature enters, it deals damage to any target equal to the number of Mountains you control. *Sparks from Purphoros's forge fill the belly of every dragon.* Power/Toughness: 5/5"