The default tiger-headed rakshasa is so iconic it can be a hard image to get out of your head. (At least it’s hard to get out of mine. Ask me to think of a rakshasa and I picture: A) Shere Khan, and B) Shere Khan in TaleSpin. Which is impressive, because I’ve never seen TaleSpin.)
The dandasuka resembles neither of
those guys that guy.
Instead, to get a sense of the dandasuka, imagine Salacious Crumb (Jabba the Hutt’s Muppet court jester) with the DIY initiative of one of Henry II’s knights (“Will no one rid me of this turbulent priest?”). Add to that a taste for gems and finery (worn) and blood (licked) and you’ve got the general idea.
Dandasukas are named (as best as I can tell) after a snake-filled Hindu hell for the envious. Given that envy is rather effective fuel for a fiend, this might explain their general hyperactivity and maliciousness. That doesn't mean they’re chaotic—they follow orders and they’re actually quite good negotiators—they’re just bloodthirsty and easily bored. (So, they’re prep school lacrosse players, essentially. (Source: I’m from Maryland.))
Assuming your group’s PCs haven’t already run into a raktavarna, a dandasuka could be a great way to introduce them to the dark world of the rakshasas. As spies and assassins (and overeager ones at that), they could be found almost anywhere in society, from the royal court to the dockyards. What might start as a simple random encounter—stumbling across a snaggletoothed gnome gnawing on the arm of a dead snitch—could eventually lead to far-off lands and the courts of the rakshasa maharajas.
An elaborately carved jeweled box is sent to an adventuring party’s patron. Out pops a comely blue-skinned genie, who promises a tale a night for a year and a day. Her tales are excellent, but she is no genie—she’s a dandasuka with her teeth filed down and a ring of mind shielding to foil alignment detection. She slays the patron the first night they are left alone together.
The local gnomish trading consortium is representing by a dandasuka. Soon political rivals begin turning up dead. Confronting the gnomes will be difficult, as they all donate heavily to the local temple of the god of revelry, and several are illusionists or wereboars besides. The gnomes and the dandasuka are links in a chain that leads to a rakshasa noble who controls most of the caravan trade from here to the Silver Peaks.
The Archbishop of Canterbury has been slain. The blame fails on four Norman knights, but they're actually innocent. The real culprit is a dandasuka. (The knights surprised the rakshasa while it was supping on Becket’s blood. It splattered them in gore and then vanished in an obscuring mist, leaving them to deal with the outcry.) The knights meant to flee north to Cumbria, but vanished en route. Adventures sent to find them will discover they are prisoners of the Heather King, a fey lord of the Yorkshire moors. Meanwhile, the dandasuka is still stirring up trouble, picking off more clergymen and doctoring the murder scenes to look like the work of a demon or sorcerer.
—Pathfinder Bestiary 3 225
I don't want to overdo the fantastic historical fiction prompts (though I’m thrilled you all like Jesuit blue dragons), but given how dandasukas are described in the Bestiary 3 the Thomas Becket one was just begging to be written.