Is Cavern Whisperer legal in Standard, Modern, and Commander?

Cavern Whisperer is legal in: Modern, Pioneer, Legacy, Vintage, Commander, Pauper, Historic.

Cavern Whisperer

Q: What type of card is Cavern Whisperer?

Cavern Whisperer is a Creature — Nightmare. Its oracle text reads: "Mutate {3}{B} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.) Menace (This creature can't be blocked except by two or more creatures.) Whenever this creature mutates, each opponent discards a card."

Creature — Nightmare

$0.27 ↗20 similarCommander, Modern

Mutate (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)