Timescoop (Vintage Paperback) by John Brunner

A Pandora's box of evil Freitas had commanded the engineers of his vast, world-wide empire to build him a device that could ransack the past. Now all the riches of the ages were his for the taking. But mere wealth was not what Freitas was after. Supreme power was what he sought, and from the past he picked the men and women who could help him gain absolute mastery over his rivals. But one thing he had not reckoned on - the power these creatures fro the past would have over him the reign of terror about to begin