Is Mortarion, Daemon Primarch legal in Standard, Modern, and Commander?

Mortarion, Daemon Primarch is legal in: Legacy, Vintage, Commander.

What type of card is Mortarion, Daemon Primarch?

Mortarion, Daemon Primarch is a Legendary Creature — Demon Primarch. Its oracle text reads: "Flying Primarch of the Death Guard — At the beginning of your end step, you may pay {X}. If you do, create X 2/2 black Astartes Warrior creature tokens with menace. X can't be greater than the amount of life you lost this turn."

Mortarion, Daemon Primarch

Legendary Creature — Demon Primarch

Flying

Primarch of the Death Guard — At the beginning of your end step, you may pay . If you do, create X 2/2 black Astartes Warrior creature tokens with menace. X can't be greater than the amount of life you lost this turn.