Dave Duncan 
Demon Sword 

All of Europe is under the control of the Khan, whose conquering armies swept across the West in 1244. Scotland, in addition, lies under the heel of England. Young Toby Strangerson, a half-English bastard, reared by a witchwife, wants only to shed his hated "Sassenach" blood and free his beloved highlands. Toby wields a sword as the outlaw Longdirk. The sword can cut down men like so many stalks of corn. But stranger winds are swirling and howling across the lochs, eldritch winds that are ridden by "hobs" and "wisps" and demons. The enemy Sassenach king is also a sorcerer. His demon soul needs a body and his Black Arts can free Europe from the Khan’s Golden Horde. This book was originally published under the name Ken Hood.