From Halopedia, the Halo wiki
Harrier-class vessels, also known as attack harriers or simply harriers, were relatively small and maneuverable third-order Forerunner warships used for interception and containment. They possessed cloaking systems allowing them to remain undetected to enemy sensors. Their armament included a form of beam weapon, as well as gravity slings which allowed them to turn asteroids or other objects into projectiles.
During the later years of the Forerunner-Flood war, the Falchion's forces stationed at the 78th Thema included 700,000 Harriers, deployed from twelve Fortress-class vessels serving as apex control stations. The harriers acted as the primary offensive element of the Falchion's fleet, launching a number of swift precision strikes against the incoming Flood ships, allowing the Forerunners to eventually emerge victorious in the battle. A large number of attack harriers, supported by numerous sentinels, also defended the greater Ark during the Flood's attack. The Keyship Anodyne Spirit posessed a number of Harriers as part of its complement.
List of appearances