Iriv, the sixth Sun-King of the Carja, led a total of four expeditions into the Daunt, hoping to break through and explore the land beyond. Eventually he (presumably) succeeded, although he and his army vanished, never to be seen again. To discourage other ventures, Basadid, his brother and successor in the Radiant Line, ordered the construction of the western fort of Sunfall and declared the land beyond it the "Forbidden West," a land where only the Sun may go. This marked the end of the Sundom's expansion.
Since then, many outlanders have journeyed into the Forbidden West; the ones who return are either near-death or driven mad by what they have discovered. The tales of these journeys differ, with some reporting to have crossed deserts of white, orange, and blue sands while others recounted vast prairies full of tall, sharp grass. Many explorers have encountered ruins of the Old Ones, even telling of a massive lake much larger than the Daybrink with an entire city of the ancients submerged within.
One notably consistent part of the varied tales from the West is the presence of new and more dangerous machines the likes of which have never been seen in the Sundom. Many visitors also spoke of strange tribes in the lands beyond the border, including drinkers of machine blood. This seems to be supported by the machine blood drinking Shaman Brin, an exile of the Banuk, who leaves the Sundom for the Forbidden West upon completion of his side quest.
According to the Nora High Matriarch Teersa, Rost journeyed to the Forbidden West in search of the bandits who killed his daughter. However, the Forbidden West remains a mysterious and treacherous unknown for the Carja.
- "Bold IRIV, who saw the Sun's passing into the West as a challenge, and forged after it with a great army, to be pushed back three times at the great canyons that would be known as the Daunt; until on the fourth time his cohort broke through, and were vanished in the lands beyond;"
- ―The Sun-Kings scanned glyph
- Mathijs de Jonge, the game director of Horizon Forbidden West, has stated that the region spans from what was once Utah to the coast of the Pacific Ocean.
- The Forbidden West's legendary status is similar to the Bermuda Triangle.
- Some of the terrain reportedly found there is possibly consistent with remnants of tactical nuclear detonations from the last stages of Operation: Enduring Victory.
- Although first mentioned in Horizon Zero Dawn, the Forbidden West is the main setting of Horizon Forbidden West.