It is on one of four elemental vendors you can unlock at the homestead, each of which has three fixed spells plus a random selection. Specifically, it is on the air elemental.
Fire: Meteor Strike(19), Immolation(13), Man-at-arms: Phoenix Dive(8)
Ice: Hail Attack(19), Mass Slow(16), Marksmanship: Arrow Spray(13)
Air: Storm(19), Make Invisible(16), Scoundrel: Walk in Shadows(2)
Earth: Earthquake(19), Petrifying Touch(8), Witchcraft: Death Punch(16)