Vrondi are air elementals. Many of them agreed to be bound into the Web-Spell with which they protect Valdemar from magic users. Magic users complain about many eyes, that keep watching them. The song "Vrondi's Eyes" refers to these eyes. Herald-Mage Vanyel who created the spell, originally intended for the vrondi to watch and notify the nearest Herald-Mage of what they had detected, but when the Herald-Mages died out, they took to simply watching instead. The constant attention eventually drives mages mad if they try to stay. The vrondi effectively kept mages out of the country for hundreds of years.
Vrondi are also averse to the emotions that accompany falsehood. This makes them excellent lie detectors. The Truth Spell is actually a spell that summons vrondi for this purpose. The first stage of the spell simply calls the vrondi. This allows truth to be discerned by whether or not the vrondi disappear when a person is being questioned. Any Herald has enough Gift to call vrondi, and can use this form of the spell. The second stage uses the inherent magic of the vrondi to compel truth. Only a Herald with a strong Gift has enough magic to make this work.