Is Alarum legal in Standard, Modern, and Commander?

Alarum is legal in: Legacy, Vintage, Commander, Pauper.

What type of card is Alarum?

Alarum is an Instant. Its oracle text reads: "Untap target nonattacking creature. It gets +1/+3 until end of turn."

Instant

Untap target nonattacking creature. It gets +1/+3 until end of turn.

Set Mirage

Rarity Common

Artist Andrew Robinson

Instant Commander Pauper Legacy