Grimfrost's Kross-Ox (Old Norse: Cross Axe) is based on an original 10th Century find from Stengstugu Hejde, on Gotland, Sweden. The head has an open section that features a cross motif. This design lessens both the weight of the axe, as well as the amount of metal required for the item, while still keeping it strong and robust enough for use.
Cross symbols were quite common in Scandinavia already during the Bronze Age that pre-dates Christianity. The cross shape could thus be a sun symbol, or even a variant of Thor's hammer.