Gate of the Dead by David Gilman
Tuscany, 1358: Thomas Blackstone has built a formidable reputation in exile, fighting as a mercenary amid the ceaseless internecine warring of Italy's City States. But success has bred many enemies, and when a dying man delivers a message recalling him to England, it seems almost certain to be a trap. Yet Blackstone cannot disobey - the summons is at the Queen's demand.
On his journey, Blackstone will brave the terrors of the High Alps in winter, face the Black Prince in tournament, confront the bloody anarchy of a popular revolt and submit to trial by combat.
And every step of the way, he will be shadowed by a notorious assassin with orders to despatch him to Hell.