A jesto-vath is a shielding spell from the White Winds School of magic. A mage can set it in advance, and then activate it as needed with a three-word trigger. It creates a warm yellow glow in a room or shelter, creating both light and warmth. A cold tent can go to balmy in moments. The only problem with the spell, aside from the necessity to cut through it to get in and out, is that it requires something approximating a roof and walls in order to put it up. Kethry puts up several of them in the early chapters of Oathbreakers.