Stargate SG1
Book 14: Valhalla

SG-1 become enmeshed in a fierce battle straight out of Nordic mythology!

Although unwilling participants, SG-1 soon find themselves fighting with the Vanir against their ancient foe - the brutal giants of Jotunheim - and feasting with them long into the night. But when dawn arrives, the team find themselves in a very different world. Although still at war, the Vanir and Jotunheim have advanced a millennia and fight with weapons of the 18th century.