A maledight is a Shanir with the infamously dangerous ability to curse, to harm with words alone. Curses are part prophecy, and part shape the future. Brenwyr appears to be the only maledight currently living.
Jame notes that there were old songs about Shanir maledights. They usually died young, killed by their families in self-defense if they did not curse themselves dead first. She speculates that the cursing talent might be related to the God-voice, but with only Regonereth, That-Which-Destroys, speaking.
Brenwyr illustrates the danger of the trait; she cursed her mother to death (or, at least, believes that she did) and she believes that her words played a part in Aerulan's death as well. Fortunately, she has been trained to iron self-control.
Jame herself, however, also exercises an ability intriguingly similar to the powers of a maledight. At the beginning of her career at Tentir, Jame joins the commandant, instructors and cadets in swearing the randon oath. She detects that one among those present is swearing falsely. Her reaction is to "swear the strongest oath she could: 'Honor break me, darkness take me, now and forever, so I swear.'" When others in the hall join her, Jame realizes that she has "struck hard" at the unknown oathbreaker. Her victim turns out to be the Randir Tempter. Also, Brenwyr seems to be incapable of cursing Torisen or Jame, since the Knorth are stronger Shanir, or her brother, Lord Brandan, who, as a blood relative, has some natural immunity.
The term is not an invention of Hodgell's, but rather a very archaic and rare English word derived from the Latin maledictus. A malediction is the opposite of a benediction; a curse, rather than a blessing.