Is Shadow of Mortality legal in Standard, Modern, and Commander?

Shadow of Mortality is legal in: Modern, Pioneer, Legacy, Vintage, Commander, Historic.

What type of card is Shadow of Mortality?

Shadow of Mortality is a Creature — Avatar. Its oracle text reads: "If your life total is less than your starting life total, this spell costs {X} less to cast, where X is the difference. *"Build your towers as tall as you want. The shadow always reaches higher." —Message scrawled on an alley wall* Power/Toughness: 7/7"