Elixir of Death

When a Norman knight named Peter le Calve is foully murdered, his severed head stuck on the rood screen of Exeter cathedral, it's up to Sir John de Wolfe, the county coroner, to find a motive for the killing. And just what is his unscrupulous brother-in-law, the disgraced ex-sheriff and Prince John-sympathiser, Richard de Revelle, trying to hide?