The Spear of Destiny — Marcus Sedgwick

Eleven Doctors, eleven months, eleven stories: a year-long celebration of Doctor Who! The most exciting names in children's fiction each create their own unique adventure about the time-travelling Time Lord.
The Third Doctor and Jo Grant are trying to track down the magical spear of Odin when they find themselves caught up in a vicious battle between two Viking tribes. But one of the Vikings is even more dangerous than he appears to be. Can the Doctor stop the spear getting into the wrong hands before it's too late?