Sergeant Simony was an Omnian with a fierce rebellion burning inside. He became a Free Chelonian, for whom "The Turtle Moves". He was the leader of a detachment of Omnian soldiers to Ephebe during the events described in Small Gods. He distrusted everyone, especially Brutha because he was a priest. Striking up an unlikely friendship with Urn, nephew of the philosopher Didactylos, Simony almost invents armoured warfare as he gets Urn (who has worked on the principles of steam-driven technology) to virtually create a WWI tank. Luckily it is never needed as Om manifests himself unto the gods at Dunmanifestin and causes havoc until his wishes are met.
Simony also almost powers a revolution as he causes Didactylos to meet with other Free Chelonians who take up the fire, much to Didactylos' bewilderment, for he has only told the truth as he saw it: he didn't expect anyone to need to fight for it. He reckoned without the hatred instilled by the priests of the Omnian religion, however.
At the end of the book, Simony is put in charge of the Quisition with orders from the new Prophet Brutha to shut it down. Permanently.
In a neat twist, Simony on Roundworld means "the buying or selling of ecclesiastical pardons, offices, or emoluments" - a grave sin against the church, or the Pope's word. Discworld's Simony is doing much the same thing: trying to bring down the church and loosen the hold of the Cenobiarch over his people (although, just as on Roundworld, the Cenobiarch is pretty much a figurehead whose cardinals and bishops hold the real power and who do things in the name of their religion that any of their followers would baulk at).
The Tank, as described, also has a striking resemblance to a sketch by Leonardo da Vinci for an armoured war-wagon. Da Vinci's theoretical model depended on several big strong men in a minimum of clothing turning cranks to make it go - however he must have been having an off-day, as the gearing system he devised would cause it to grind to a halt, as the two teams would have been working in opposition to each other and would have cancelled each other out.
One wonders if an otherwise disregarded marginal sketch by Leonard of Quirm might have been the inspiration...