The Stone Guardians are living statues of stone that guard Cathedral zu Heltzer and other ancient sites in the Echovald Forest. They can surprise the unaware travelers in the Forest as they appear from a distance, to be stoic statues of marvelous art, but when players come near them the statues will crumble leaving behind living creatures of stone that attack viciously.
Stone Guardians resemble Greek and Roman sculptures. The Kurzicks who dwell in the Forest insist that the monuments that these creatures guard predate them in the forest and were not made by their ancestors. However, they also insist that the structure many of them guard, Cathedral zu Heltzer, was built by their ancestors. Additionally, Terrik zu Heltzer, and presumably other Stone Singers, have the ability to create exact replicas of Stone Rains called zu Heltzer Guardians.
Whenever these statues ambush the party, they come from pillars or other stone objects that have a miniature of the creature somewhere in the sculpture, usually on top. The miniature matches the creature to be spawned, making it easy to predict their surprises.
Temple Guardians are the exceptions to the above. There is no history behind them, they were most likely just created by Canthan architects and enchanted to defend things.