The Web-Spell laid on the land of Valdemar acted as an early warning system, sounding an 'alarm' whenever anything was seriously wrong. The spell required four Web-Guardians, one for each quarter, to feed power into the spell and monitor it for alarms.
During the reign of Randale, Herald-Mages were slowly decimated without being replaced. Before long their were only four Herald-Mages in the entire kingdom who had sufficient power to serve as Web-Guardians: Savil, Kilchas, Lissandra and Vanyel.
For many years Vanyel had been studying the problem, working out a way to power the Web-Spell without Guardians. He slowly enhanced a node lying beneath the royal palace, intending to use it as the main power source. In place of four Herald-Mages, he tied in the entire Heraldic Circle, knitting every single Herald into the Web via their Companions. He also intended to tie in a number of elemental creatures to help monitor and strengthen the network.
With time running out, Vanyel performed the spell to remake the Web. Tying in the node created a small Heartstone in the basement palace Work Room. With the Companions' help, he spread Guardianship among all the Heralds. He then invited the vrondi, small air elementals, to take part, offering them a magical energy "food" supply in exchange for acting as lookouts, watching for strange mages. Unfortunately, he didn't have the time to go back later and add more elementals of other types.
Though knowledge of the spell, along with the Heartstone, were lost when true magic vanished from Valdemar, the effects of the web continued for another two hundred years. The Web-Spell was what gave the Heralds the knowledge of the deaths of other Heralds. The faithfully watching vrondi were what drove foreign mages into insanity if they stayed too long in Valdemar.
The Web continued until the Mage Storms during the reign of Selenay. The spell had been steadily weakening, and with the training of Princess Elspeth, Herald-Mages returned to Valdemar. At that point, the spirit of Vanyel, still guarding Valdemar's northern border, took down the Web-Spell before a Mage Storm could shatter it.