Herald was a Lambda-class T-4a shuttle used by the Rebel Alliance during the Galactic Civil War.
According to the plan by Crix Madine and the Sullustan technicians who had just joined the Alliance, they needed a warhead in order to destroy the Imperial-class Star Destroyer Invincible and end Operation Strike Fear. The Rebels soon learned about a new warhead being transferred from Celanon to a deep space transfer zone by the BFF-1 bulk freighter Gafra and planned to attack it at its destination.
An initial Rebel attack left the Gafra damaged and stranded before withdrawing. However, the freighter was still protected by a field of mines around it. Keyan Farlander in a Y-wing cleared them out and disabled the freighter. When that was done, the Herald came and boarded the freighter. Imperial transports came and attempted to destroy the freighter before it could fall into enemy hands, but Farlander was able to drive them off.
With the help of abducted Cygnus Corp staff, the warhead was then delivered to the Invincible in a shuttle modified to appear to be an Imperial vessel, and the Star Destroyer was blown up from the inside.