" There is no number so unlucky as thirteen. Once, in Valhalla, there was a feast for twelve gods, but Loki, the trickster god, went uninvited and he played his evil games, persuading Hod the Blind to throw a sprig of mistletoe at his brother, Baldur. Baldur was the favorite god, the good one, but he could be killed by mistletoe and so his blind brother threw the sprig and Baldur died and Loki laughed, and ever since we have known that thirteen is the evil number. Thirteen birds in the sky are an omen of disaster, thirteen pebbles in a cooking pot will poison any food placed in the pot, while thirteen at a meal is an invitation to death. Thirteen spears against a fortress could only mean defeat. Even the Christians know thirteen is unlucky. Father Beocca told me that was because there were thirteen men at Christ’s last meal, and the thirteenth was Judas. "

― Bernard Cornwell , Lords of the North (The Saxon Stories, #3)