A patchen is a hate-filled, horrifying creature with—
Wait, I got that wrong.
A fachen is a hate-filled, horrifying creature with one eye, one arm, one leg, and one mouth full of terrifying teeth. (Seriously, just looking at it risks a fear effect.) It is a creature out of a campfire tale—the cautionary variety, where the child who strays off the path doesn't make it out alive.
While Paizo’s fachens are aberrations, they come from Scottish folklore, and fey-themed adventures set in the highlands and burrens (or just over into the Otherworld) are still probably the most natural fit for fachens. (And if you need to beef up the CR, the fey creature template wouldn’t be a bad way to go.) But I could easily see them in a Gothic horror campaign as well (particularly if you need a good nighttime scare for a party that’s not yet up to werewolves and dullahans) or in a truly weird underdark (see the “Azruverda” and “Dossenus” entries for more in that vein).
I mention faeries and fairy tales a lot in this blog—at least three times already just this entry—but I rarely talk about fables. Thanks to their nearly impossible forms, there’s a surreality about fachens that recalls The Voyage of the Dawn Treader, Gulliver’s Travels, and the Earthsea novels. I’m not suggesting your adventures should be fables themselves—99% of the time adventures should serve the group, not some lesson or allegory—but if you’re looking for that more magical, free-floating, anything-might-be-on-the-next-island feel, fachens are a perfect way to go.
A pukwudgie has stolen Judge Stormtower’s infant son. The judge turns some adventurers out of prison to find his heir before the boy is consumed. The crafty pukwudgie makes sure his escape route intrudes on a fachen’s territory, expecting that the hate-filled beast will make short work of any pursuers.
A wise woman foretells that adventures will “face a fear that is not their fear nor their face” after following “in the footsteps of a man who has never walked.” At the next island they come to, a fachen strikes. Hidden among the fachen’s treasure is a single magical boot that will nevertheless prove useful as the party’s journey progresses.
Fachens who dwell in an area long enough will be assigned a name and personality by the locals. Hoppin’ Jax haunts at least three different crossroads, drawn to the cries of lawbreakers staked there to die of exposure. (As he mostly attacks criminals and travelers, the locals regard him almost fondly, though still with a shudder and the sign against evil.) The Blade of Bile hacked up three King’s Guards last spring; his pursuers lost him down the Stone Giant Stairs and the bounty on his head is as yet unclaimed. The orchard-destroying Old Marg was finally brought down by hunters a few years back after it killed the earl’s prize peacocks. Yesterday morning a fresh peacock body was found, slain exactly the same way as Old Marg used to…
—Pathfinder Adventure Path #63 88–89 & Pathfinder Bestiary 4 95
Pathfinder Adventure Path #63: The Asylum Stone has the full entry on fachens.