Rick Priestley, also known as "Based Priestly" on /tg/, was once an employee of Games Workshop early on in its history. He wrote the early storyline for both Warhammers, and as such is considered their creator; the genesis of Fantasy was his take on generic fantasy settings given a touch more grounding, and 40k began with an existing setting idea of his combined with the themes of Paradise Lost. He was one of the designers of the actual rules of Warhammer Fantasy Battle along with Bryan Ansell and Richard Hailliwell, and also designed WFB's large-scale counterpart called Warmaster. Rogue Trader was developed as a retooling of a boxed game he had created which had been largely unsuccessful. By July 1997, he and Andy Jones and Marc Gascoigne had developed the idea of the Black Library, Games Workshop's publishing arm, and the Inferno! magazine dedicated to their Specialist Games line. The Lord of the Rings Strategy Battle Game was his last GW game, designed to steer the company back in his preferred design direction.
He left GW in 2010, ostensibly due to a change in corporate culture (read: GW caring more about fleecing their playerbase than making fun games). Since then he has worked with Warlord Games on Bolt Action, Hail Caesar, Black Powder and Beyond the Gates of Antares alongside other former GW game designers. His legacy is generally well-regarded by most players, though his ideas on and suggestions for the setting of both Warhammers got pretty questionable or just plain weird as time went on, kinda like George Lucas with Star Wars.
He recently released the fantasy wargame Warlords of Erehwon, which is loosely based on the rules in Bolt Action and Gates of Antares, and is great.