Most swarms aren’t all that interesting, in and of themselves, aside from whatever special feature (disease, contagion, wounding, etc.) they’re packing. But hellwasp swarms ooze (buzz?) intriguing plot hooks. First of all, they’re from Hell…which means either the PCs have a reason to be on their turf, or they have a reason to be on the PCs’. Second, even other fiends hate them. Third, they’re intelligent—a swarm with a hive mind that goes out of its way to insult you as it’s stinging you to death is pretty novel. Fourth (and most interestingly) they can inhabit both the living and the dead—turning friends into (usually short-lived) unwilling slaves and corpses into horrible vessels. Which raises the question of “Why?”…leading to the fifth and final tick on our checklist: hellwasp swarms may or may not be the dispersed soul of a greater devil…
In one of her first adventures, a cleric and her friends defeated an accuser devil servant of some entity called He Who Is All. Later, she saved an inquisitor rival from a group known as the Legionnaires. Clues from that encounter point to a long-lost infernal duke of swarms…a suspicion confirmed when she is assaulted by a hellwasp swarm during her research.
Entering a lonely inn—kept dark even in the middle of the day—adventurers find a surly married couple—actually two hellwasp swarms in disguise, with their hosts near death. If they defeat the threat, the company may be able to save the couple. They may also find the tiny rift in reality that leads to a seemingly identical bar in the infernal city of Dis.
A hellwasp swarm has mutated. This more docile, less voracious breed seems compelled to emulate humanity, attempting to continue its hosts’ former lives even after inhabitation. (For some reason, they seem especially taken with bankers—perhaps for the order and reserved remove with which they conduct their lives.) If the dumb show is revealed for what it is, though, the threatened swarm still attacks with all the ferocity of its forebears.
—Pathfinder Bestiary 3 146
One final thought: hellwasp swarms are likely particularly beloved of whatever your fantasy world’s equivalent of the Lord of the Flies is—Baalzebul on Golarion, or Beelzebub, etc.