The Watchers are Archeons who work with Eredar to watch over the Otari of the Tower. 

From the tome of the Arts:

The magic users of Uteria are watched over by the Archeons, or more commonly called; the Watchers. After the war against the Warlock King, Olaekin Baleband, the last of the wizards who served the United Kingdoms (Aradan, Norhand, and Endamas) formed the Council and began to put forth rules to try and stop the wide use of ruinous magic. The formed the Archeons and trained them in ways to stop magic. Over the 700 years since the end of that war, the Archeons have become quite powerful, not just traveling the lands, but also keeping an eye over all the tower to make sure none begin to use forbidden magic. There are many different factions of Archeons, those who travel the lands hunting Nulthari (The Hunters, called witch hunters by commoners), those who keep an eye over those in Eredar (the Order of the Tower), and those who pair up with an Otari wizard as a guardian and partner (The Keepers).

They have the ability to nullify magic, as well as cut a person off from the energies around them. This takes a great amount of power, and often it takes many Archeons to take down a powerful Nulthari, and even more to permanently pacify a Nulthari.