The Saga Annika refers to at the beginning about Valkyries and their looms is Njal's Saga.
The runestone that Sigi places at the foot of the cliff says 'FREJA' in the Elder Futhark.
The "runic alphabet" (there were variations at different periods and places) is called a Futhark.
Nicola Walker and Shauna Macdonald previously worked together on MI-5 (2002), playing Ruth Evershed and Sam Buxton, respectively.