Modern fairy stories (and I’m especially including fairy films in this) have gotten really good at That Moment. It’s That Moment when the fairy tale becomes a faerie plot. That Moment when the trap is sprung. That moment when the Pale Man awakens or the monster of the month transforms behind Hellboy’s back and attacks.
Spriggans are ideal for That Moment in your game. They’re harmless, dirty gnomes—until they’re not little or harmless at all.
Why spriggans transform is up to you and your worldbuilding. They’re practically fey, so transforming might just be what they do—likely as a result of some original sin or curse handed down by Oberon, Rhiannon, or another great power. In Golarion, spriggans are gnomes who went too far in their efforts to rid themselves of the Bleaching. Like trolls, spriggans might also be the link between gnome- or faeriekind and giantkind, with gnarled feet firmly planted in both worlds. Or they might even be mutations, science experiments, alchemical super-soldiers, or just creatures of rage whose ire takes very physical form.
Davis Jagger is a bloodthirsty quickling who wants revenge on the sheriff who drove him out of Litchfield. After a speedy (naturally) search of the local abandoned mines and barrows, he rounds up the Ox Mob Seven as his heavies. These foul spriggans, known for the rings they wear through their noses, disguise themselves as gnomish tinkers to get into town. They wait until the gates are shut and locked for the night to set flame to the prison, the inn, and the summoning tower.
Adventurers travel through a strange fey dungeon, each room of which seems to be a puzzle—or a trap. The second chamber features a half-dozen unkempt gnomes (actually spriggans) eating in grizzled silence around a dining room table, upon the surface of which has been carved, “Arrive and leave with grace.” To successfully leave the room, the adventurers must sit at the empty seats and say grace or otherwise bless the food or the company. Doing so repels the spriggans, who file out of the room, leaving the door open for the party to follow. Otherwise, the spriggans continue eating and do not respond to any form of speech. Anyone who samples the food without prayer is magically compelled to join the gnomes in their feast. If anyone physically touches the spriggans or tries to force the locked door, the angry small folk howl in rage, swell in size, and attack.
Elephant graveyards are guarded by the mad-bone men, gnarled, ash-gray spriggans who ride on hyenas, play talking drums, and dine on the flesh of the fallen beasts. They attack all who trespass on their land, swelling to giant size and fighting with ivory-studded greatclubs.
—Pathfinder Bestiary 2 257
Paizo clearly loves spriggans; they’ve shown up in everything from Adventure Paths to modules (with a stronger variant in Realm of the Fellnight Queen) to Gnomes of Golarion. There’s even a mythical spriggan city mentioned in Heart of the Jungle.