Read the novel in the late eighties as a child. Short novel in a paperback format. About 150 pages. My memory is hazy so some details may be plain wrong.
The evil wizard X, left the realm of (civilized) man into the wild. Alternatively, he was always one of the barbarians. He found a magic trinket (or knowledge) that gave him serious juice. Raised a barbarian horde and mounted a campaign to conquer the kingdom.
He first laid siege to a castle of said hero. Some minor border noble. Possibly a baron. The baron saw his own wizard get killed by the power of the wizard. I think this may have been in the end and not the beginning.
The baron left to find help (from the king?). He passed through other baronies/counties. Picked up the daughter of a noble on the way. Worried about getting permissions to pass through unfriendly baronies.
Learned a dangerous spell along the way the could kill someone or himself.
Failed in the mission. Came back home. Saw the battle going the wrong way. Did the spell. Won the day.
Cover was maybe the baron holding a sword, the chick (scantily clad), wolves??? and snow. IIRC, the setting was arctic.