- The Last Viking (2008)
- Riders of the Ice Horse (2009)
- The Sword and the Spear (2010)
- The Blood of the Sun God (2012)
- The King's Shadow (2013)
- The Queen's Vengeance (2014)
- The Empire's End (2015)
- The Dawn of Vengeance (2017)
- The Day of Doom (2018)
- The Fall of the Empire (2019)
Achen has also written several short stories, including "The Last Berserker" (2011), "The Battle of the Ice" (2012), and "The Sword of Sigurd" (2013).