Is Dulcet Sirens legal in Standard, Modern, and Commander?

Dulcet Sirens is legal in: Legacy, Vintage, Commander.

What type of card is Dulcet Sirens?

Dulcet Sirens is a Creature — Siren. Its oracle text reads: "{U}, {T}: Target creature attacks target opponent this turn if able. Morph {U} (You may cast this card face down as a 2/2 creature for {3}. Turn it face up any time for its morph cost.) *What are the orders of a king or the demands of a creed next to such melodies?* Power/Toughness: 1/3"