Udilf was the leader of a town in eastern Europe sometime in the early 11th century.
Udilf and Mang had been business partners. Udilf had the ability to know what people needed, and Mang was a skilled craftsman. When the town needed a new leader, they chose Udilf. Mang disagreed with the decision, but treated Udilf with respect.
When a race of "creatures" began attacking nearby villages, Udilf prepared the town for battle. The townspeople made weapons and armour.
Udilf welcomed the Fifth Doctor and his companions when they arrived at the town. He didn't agree with the Doctor that the creatures could be reasoned with, but he allowed the Doctor and Adric to try. When they returned with the news that the "creatures" were merely humans of different appearance, he allowed the Doctor to contrive ways of scaring the invaders off.
The Doctor's plans failed to stop the invaders, however, and Udilf was one of several who died in the attack. He died outnumbered but with a sword in his hand. (PROSE: The Immortals)