Baron Maupertuis was a wealthy 19th century criminal.
Maupertuis was Dutch and owned the Netherlands-Sumatra Company. Around 1887, he became a naturalised British citizen. He attempted to be extremely British by doing things like making friends with the Prince of Wales and going to Ascot. Mycroft Holmes and his associates suspected Maupertuis was engaging in untoward business arrangement, so they invited him to join the Diogenes Club to keep a better eye on him.
A member of the Library of St John the Beheaded, he and his servant Surd stole the journals of Siger Holmes to obtain the chants they contained which could open dimensional gateways. He planned to invade and conquer the planet Ry'leh for the British Empire.
However, Maupertius was being manipulated by Sherringford Holmes to aid in the release of Azathoth. Maupertuis' army would distract the Shlangii guards, letting Sherringford move Azathoth to a location from which she could be taken to Earth and convert the population to her servants.
Colonel Warburton, surprised and frustrated by the Shlangii's resistance which was greater than he had been led to believe, argued with Maupertuis, blaming him for the misinformation. Enraged at the Colonel's failure to defeat the natives, Maupertuis killed Warburton. John Watson, who was in the same camp, was almost killed as well by Maupertuis. Fortunately, Ace arrived in time to kill the Baron first. (PROSE: All-Consuming Fire)
Behind the scenes
Baron Maupertuis was a character mentioned in passing at the beginning of the Sherlock Holmes story The Adventure of the Reigate Squire.
Andy Lane, author of All-Consuming Fire, used Maupertuis again as the main villain in his Young Sherlock Holmes novel Death Cloud.