The Elixir of Death by Bernard Knight
1195. Prince Johnplots to seize the throne from his brother, Richard the Lionheart - and in his wicked schemes he is supported by King Philip of France, who offers to help John financially by sending him a mysterious alchemist, a Mohammedan named Nizam, who claims to be able to transmute base metals into gold. But the ship which was transporting Nizam and his retainers to the Devon tin mines is found wrecked off the south Devon coast, its crew savagely slaughtered. Shortly afterwards, Sir Peter le Calve, a Norman knight living near Exeter, is foully murdered, being part-crucified and beheaded, his head stuck on the rood screen of the cathedral. It's up to Sir John de Wolfe, the county coroner, to find a motive and connection between the killings. And just what is his unscrupulous brother-in-law, the disgraced ex-sheriff and known Prince John-sympathiser, Richard de Revelle, trying to hide?