She attempted to trick Amedahast, the first apprentice royal magician, into aiding an assassination attempt on the young Azoun I. Her assassins were defeated by both Amedahast and Azoun I, and she quickly fled Cormyr.
As a ghazneth, Lady Merendil had the power of disease, causing a feverish plague to sweep around her, sapping the strength and will of her opponents. Even if her enemies survived their encounter with the deadly demon, the plague weakened them enough that the ghazneth could quickly overwhelm them later on.
Only called "Lady Merendil" in Cormyr: A Novel, in Beyond the High Road, Merendil was identified as "Mirabelle Merendil". Death of the Dragon, its sequel, later used the name "Ryndala". The correct name is confirmed in Ed Greenwood's "The Merendil Gold" article in Dragon #409.