A writ is a court order directed from a higher court to a lower court or government official to comply with specific instructions. A writ is usually only permitted when a defendant has exhausted all other remedies. This means that all appeals must be attempted prior to seeking a writ. Often, defendants use a writ as a means to raise issues not contested in an appeal such as ineffective assistance of an attorney or prosecutorial misconduct.
Writs can be very complex and typically involve very specific legal details that were not raised during the original appeal. For this reason, you need a seasoned attorney to assist you in requesting a court to issue a writ.