Harpoon nuclear missile
From Halopedia, the Halo wiki
The Harpoon nuclear missile was to be a United Nations Space Command starship-mounted thermonuclear weapon. It is mentioned in a piece of concept art for the UNSC corvette Andraste that Isaac Hannaford designed for Halo 3, though it was not featured in the game.
It shares its name with a modern-day anti-ship missile.