This article covers a topic that has not been fully explored on the show and thus contains some information taken from the books, in addition to information from the TV series.
The encanto is the unique ability of vampires to almost hypnotically convince other humans—mundanes, Downworlders and Shadowhunters alike—of almost anything and to persuade them into doing almost thing they're told.
Performing the encanto is a skill practiced by vampires and developed over time. Meanwhile, Shadowhunters undergo training to resist it. The use of soothing voices and prolonged eye contact on their victims contributes to the success of the encanto.