Daemati are faeries that have the ability to walk into another person's mind. They are able to read, influence, and even shatter someone's mind, depending on their training and power.
When Feyre is rescued by Rhysand on the night of Calanmai, they make a small connection. Rhysand uses his Daemati powers to read Feyre's mind and deduce that she is lying about her name in Tamlin's residence when he discovers she was being glamoured by Tamlin.
He later uses his ability in Under the Mountain to interrogate a faerie from the Summer Court and kill him, as a mercy.
work in progress