Charlemagne's legal code 'Capitulatio de partibus Saxoniae', from the year 785 AD, states that a crime "If someone at springs or trees or groves should make a vow or sacrifice something in the pagan manner and eat in honor of demons" -- the demons being pagan gods Odin, Thor.
In the article “Pagan Survivals and Syncretism in the Conversion of Saxony” (Karras, R. M. (1986) Catholic History Review, 72, 553-572) provides additional detail where "The connection of pagan sacrifice with trees, as mentioned in the Capitulatio, is definitely found in Saxony. The Indiculus mentions 'the rites of the woods which they call nimidas.' The sacred oak of Geismar in Hesse, destroyed by Boniface, is a Saxon example of the veneration of trees practiced by many Germanic peoples. When Charlemagne conquered Saxony he destroyed or struck down the Irminsul, a Saxon idol or object of worship. Etymology as well as contemporary description indicates that the Irminsul was a pillar or column.” A historical source of the column is from Rudolf of Fulda, a Benedictine monk writing in the 9th century AD, where he referenced that the Irminsul was a trunk of wood, a universal column “as though it were supporting all things”.
