Acheron (pronounced ASH-uh-ron) is an arctic land found north of the Fremennik Province that is not currently accessible to players. It was suggested that Acheron could be part of a future fourth quest in the penguin quest series by Mod Osbourne in a Behind the Scenes video for Some Like It Cold in August 2012. Acheron is referenced in-game:
- The mammoth iceberg that is inhabited by acheron mammoths was once part of Acheron.
- After Some Like It Cold Chuck says that Larry believes that the penguins' secret base is located on Acheron.
- At Balthazar Beauregard's Big Top Bonanza Sergeant Damien can be seen saying, "I don't know what I've been told, but Acheron is mighty cold!"
- Brundt the Chieftain and some of the crew on Lady Zay mention Acheron. According to Brundt, the climate is so bitterly cold that few can survive in it, and players will need an agile body just to live.
- In Postbag from the Hedge 20, Brundt states that the Fremennik will lead an expedition to Acheron soon.
- In ancient Greek Mythology, Acheron was known as the river of pain, and was one of the five rivers of the Greek Underworld.
- Various species native to Acheron (i.e. penguins, polar bears, walrusses and mammoths) are intelligent and capable of human speech. The source of this is unknown.
- Before the release of The Tale of the Muspah, some players speculated the ritual site was on Acheron, although with the release of Ritual of the Mahjarrat quest, it is known to be in the ritual plateau.
- The Rune Essence mine might be located on Acheron, as it is located north of the Wilderness and Lunar Isle.