If you couldn’t tell from the string of L’s, R’s, CH’s, and A’s, the alpluachra is a newtlike faerie from Irish folktales. Should it catch you sleeping by the river, it promptly slithers down your throat. There it becomes a parasite, eating all the food you consume and slowly starving you to death unless you douse it with salt. Charming, right?
(I can easily imagine how tales of the alpluachra got started. Having failed to get St. Patrick to drive out the newts along with the snakes and frogs, we Irish apparently just decided to badmouth and scapegoat them instead. A bunch of herpetophobes, we are.)
The alpluachra is a great nod to folklore, but it's not really a high-adventure kind of encounter…more like an incidental happening at a stop along the road, or a solo encounter for one player when the rest of the group is absent. Killing an alpluachra is a nice way to earn some cred with the locals, to be later cashed in for rumors or favors. And it’s just a nice way for new adventurers to begin engaging with the magical world.
The hard part is spotting the alpluachra before it implants or diagnosing soon it after. Otherwise the fey could quietly haunt its victim for weeks without him or her knowing. Then again, if a parasitized PC drinks the wrong potion, the effects could be immediately amusing and/or dramatic. (Let’s just say it’s a good thing that the enlarge person spell description specifies that it only works on humanoids.)
And then there’s the matter of alpluachras speaking not Sylvan, like most fey, but the dark tongue of Aklo instead…
An alpluachra haunts the common room of a riverside taphouse. So much time spent in the gullets of barge workers has gotten it addicted to pipeweed. If its current host doesn't partake, it will sneak out late at night to pilfer unfinished cigarillos and perhaps find a new throat in which to dwell—which may afford adventurers extra opportunities to spot the fey.
A dwarf elder is starving to death. An elf armchair physician suspects an alpluachra, rightly assuming most dwarven caregivers would be unfamiliar with the woodland fey. If adventurers can convince the dwarf to try the saltwater cure, they may both save his life and usher in a thawing in dwarf-elf relations…assuming the cure doesn’t kill him.
A leprechaun hears a group of adventurers nervously plotting their first trip below the surface. He introduces himself just as they are lamenting their lack of firsthand knowledge. For a certain amount of gold, he promises to supply a guide who speaks both Common and Aklo. Only after they’ve agreed does he produce said guide, a particularly intelligent alpluachra, and reveal the “one little, ahem, rider” in the contract.
—Pathfinder Bestiary 4 9