D&D is a poor fit for this style of game, simply because D&D is high magic and high magic takes a hammer to traditional non-magic intrigue scenarios in short order. This is similar to how intrigue scenarios in futuristic settings ultimately end up turning into outgrowths of the hacking system - because everything has become digital - intrigue in D&D becomes magical intrigue and spell vs. counterspell.

There are any number of games that are much better for this kind of sword & skullduggery approach to gameplay. Beyond specific examples this falls quite well within the FATE Core design space (Fate core is full of allusions to the Nehwon stories of Fritz Leiber - which include many examples of the kind of campaign you're describing - for a reason), and Fate is available for free.