"The Lay of Hakon" (6.4.31): Leif Longfoot's song celebrating the victories of the Viking ships is based on this Old Norse poem. (Source.)
"The Unquiet Grave" (5.7.22, 5.8.11): Merle recites one half of this poem about a lost lover and Erik(a) finishes it from beyond the grave. (Source.)
"Hans Carvel" by Matthew Prior (1.7.21): Merle quotes from this 18th-century English poem when she sees Tor—"Forthwith the devil did appear for name him and he's always near." (Source.)
Domalde (7.4.4): King Eirikr's sacrifice is based on the old Norse legend of Domalde, the Swedish king who allowed himself to be killed to save his people after three years of famine.
Midvinterblot by Carl Larsson (4.8.30-47): The painting that Eric Carlsson creates is based on a real-life painting that hangs in the Swedish National Museum to this day.
Supermarine Spitfire (3.1.4): David's shot-down plane was an actual single-seat fighter aircraft used during WWII.
Pop Culture References
Six Degrees of Separation (1.1.16): The OneDegree app is named for a simplified version of the theory of six degrees of separation.
"Stairway to Heaven" by Led Zeppelin (7.5.42): Eric Seven gives a little shout-out to this song as he's about to die on the stone table—"Merle. My spirit is crying for leaving."
The Rite of Spring by Igor Stravinsky: The chapter titles in Part 7 happen to correspond to some of the dances from this 1913 ballet. It's fitting since this orchestral work is also about sacrifice.