Zaras cast an illusion on one of his apprentices to look like Raschar going on a three-day hunt with the rest of the Court. Idra, of course, tried to take advantage of his 'absence' to leave Petras. Raschar and Zaras waited for her at the stables, where Zaras hit her with a spell that knocked her out. While she was unconscious, he took control of her body. When she woke up, he forced her to ride out to an old tower, where Raschar imprisoned her, tortured and raped her, eventually ending in her death.
Kethry used her magical mirror-egg spell on Zaras and his apprentices during the Rethwellan coup, which took them out of the fighting. Zaras' fate is never mentioned, but given how things turned out for the others involved in Idra's death, it was probably fatal.