The Charmstone, also known as Isluthsia, is an Overstone that makes its victims extremely susceptible to suggestions and commands from the wielder. According to Elden LeBlanc it has "brought mighty kings to their knees".
It's first known wielder was Elden Le Blanc, the magical advisor to Lord Quandel. LeBlanc first use its power on Quandel's son Ben Quandel as a demonstration to his student, Colindra Montaine, much to Colindra's displeasure.
He later attempted to use it to forcibly seduce her, but was killed when Colindra rocked the Banestone and struck him with the Banestone's energy blasts in self defence. Unfortunately, this allowed G.A.T.E.S. to possess Colindra through the Banestone and forced her to take possession of LeBlanc's stones, including the Charmstone.
Under G.A.T.E.S.'s malign influence, Colindra would use the stone several times to bend others to her will.