He comes from the second branch of Lorraine-Vaudemont, ducal family of Lorraine and therefore related to the Guise. He married, in 1579, Marie of Luxembourg, Duchess of Penthièvre, heir to this Breton Duchy, which gave him, among other things, a justification for being appointed Governor of Brittany in 1582, by his brother-in-law, the King of France Henri III. The latter had indeed married Mercœur's sister, Louise de Vaudémont. Having become head of the League after the assassination of the Duke of Guise…