- “If anyone is to do any slaying, it will be me.”
Prosper is a high-ranking nobleman in Orlais, inheritor to the dynasty of Ser Gaston de Montfort, who rose to prominence during the Fourth Blight. He is a close personal friend of Empress Celene I; he has a son, Cyril.
Prosper spends most of his time outside of Orlais on business or vacation, styling himself as a duke. He is known for throwing very extravagant parties, and regularly hosting wyvern hunts in his mountain estate in the western Free Marches, Chateau Haine, which his ancestor acquired after the Fourth Blight.
He also appears to be fluent in Qunlat.
Prosper is a major character in Mark of the Assassin. According to Tallis, he holds the "Heart of the Many", supposedly a gem which he has no right to possess. Tallis enlists Hawke's aid to infiltrate Prosper's home, the Château Haine, and steal the Heart of the Many.
Hawke and company arrive at the Château, having apparently accepted Prosper's previous invitation to the Champion of Kirkwall. After slaying a wyvern during a hunt, Prosper welcomes the Champion into his home. But as Hawke and Tallis infiltrate his vault, it is revealed that Prosper knew who the latter was the whole time and had the two imprisoned. As Tallis explains to Hawke, there never was a jewel. She was in fact after a Tal-Vashoth named Salit, who was going to sell confidential information about the Qunari to Prosper.
After the two break out of their cell and if Hawke and company attempt to escape right through the Chateau, Prosper will bar their escape as he summons a brigade of chevaliers and harlequins to fight them and seals the Chateau. He then leaves the Chateau, ordering his Chasind bodyguard, Cahir, to deal with the interlopers.Prosper then met with Salit, expressing his regret for agreeing to spare Tallis and Hawke and insisted they get their exchange over with. Instead of the blackpowder formula, dreadnaught plans, or a map of Qunanadar, however, Salit gave Prosper a list of names, detailing the various Qunari agents in Thedas, which Prosper failed to see the significance of and was angered by the seemingly useless offer. As he was confronted by Hawke, he unknowingly passed the list to a disguised Tallis. After all parties exchanged words, Prosper fires a green substance at Salit that attracted his wyvern mount Leopold, who killed him. In the ensuring battle, Hawke manages to trip Leopold while Prosper is riding him in mid-charge, sending the wyvern over the cliff but Prosper was clinging to the edge for his dear life. He promised that Empress Celene would burn Kirkwall to the ground in revenge for his death. Diplomatic/Helpful Hawke will attempt help Prosper up but the Duke would attack with a dagger, forcing Hawke to drop him. Sarcastic/Charming Hawke will simply walk away from Prosper as he loses his grip. Aggressive/Direct Hawke would step on Prosper's hand. Regardless of Hawke's response, Prosper is sent falling to his death. However, his threats were futile, as it would become scandalous if it became public knowledge that he was dealing with Tal-Vashoth, possibly on the Empress' orders.
- Prosper shares his family name with the Simon de Montfort, 5th Earl of Leicester and his son Simon de Montfort, 6th Earl of Leicester the former who died leading his soldiers in the Albigensian Crusade; and the latter lead the Barons to rebel against King Henry III, and called for the first directly elected parliament in medieval Europe, respectively.
- After the wyvern hunt, Duke Prosper talks about how hunting a wyvern is "a terrible risk. But the prize..." This is a reference to much-maligned line from Mass Effect 2, during a female Shepard's romance path with Jacob which Mass Effect fans had much fun with.