Unicorn Valley was a name given to an area east of the River Lum, which today consists of the path between Varrock and Gunnarsgrunn. As its name indicate it was an area where numerous unicorns lived. The term itself is only used in Armies of Gielinor, a Funorb strategy game created by Jagex.
During the God Wars it was the location of a Zamorakian fortress ruled by Pazuzu the Pyrelord. Saradomin launched an attack on Pazuzu as he attempted to retreat back to his fortress after his forces had been pushed back in numerous battles before. Despite another loss at the hands of Saradomin's forces Pazuzu survived the encounter.
Today the area is forested and inhabited by numerous goblins with little to no indication of the fortress that once stood here, there are no unicorns found in the area, the closest being found on the west side of River Lum or south of Varrock.