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Xerfilstyxes were insane baatezu that blending the features of devils and slugs and acted as the self-appointed guardians of the treacherous River Styx. The broken minds of the horrifying creatures were filled with thoughts that were not their own, and they were intent on stealing more memories for their own self-destructive pleasure.
Xerfilstyxes stood about 12 feet (3.7 meters) tall and weighed 1,500 lb (680 kg). From the waist up they resembled "classic" devil, with a muscular humanoid torso and arms, large hooked claws, and massive leathery bat wings protruding from their backs. They had large, fang-filled mouths, and two big black horns that curled out and up from their foreheads. From the waist down, however, a xerfilstyx had the body of a slug, a pulsing mass of flesh that dragged along the ground and left a slippery trail of slime behind.
The top half of a xerfilstyx's skin was near-translucent and sickly blue, while the bottom half was more purplish-blue. Vividly visible and present across their entire bodies were massive veins that pulsed blood red, yet their natural coloring still made them adept at hiding under water.
Xerfilstyxes were completely insane, the long-standing result of their constant exposure to the Styx. They swam frequently in its oily depths and whenever they did they were constantly blasted by barrages of drained memories from lost souls that had wound up in the Nine Hells. Despite the mental damage it caused, the xerfilstyxes seemed to enjoy the process, actively sampling the frightening memory fragments and deriving much gratification from bombardment even as it dissolved their minds.
Xerfilstyxes believed that other creatures should also sample the waters of the Styx, yet they paradoxically took umbrage with anyone that tried to access the waters and attacked them with furious vengeance. The insanity of xerfilstyxes made them highly unpredictable and they cared little for the fate of the world, only in whatever they came across that they could take and destroy.
Despite all this however, xefilstyxes were still uniformly lawful evil; though contradicted by their rage, they still felt it was their duty to help reveal that all should sample the Styx. They were also not to be mistaken for stupid, their extended time in the memory-ridden river making them some of the most knowledgeable beings in the Nine Hells (even despite their often lacking capacity for applying that knowledge). Raving lunatics that they were, the madness of the xerfilstyxes nonetheless followed a method.
Xerfilstyxes were not debilitated by the deleterious effects of the Styx, and could not be hurt by the cold, mental effects, and attacks that would pierce their bodies. They could exude an aura of fear within a 30 feet (9.1 meters) radius around them. Nearly every part of them, from their fangs, claws, goring horns, and slug-like tails could be used as a weapon. They were also dangerously mobile, being fast on land, excellent swimmers, able to burrow underground, and fly quickly (if poorly) through the air.
If striking an individual with their tails, it was possible for xerfilstyxes to attach themselves to their opponents, after which they could physically and mentally drain them through the many small suckers covering their tails. Befitting creatures native to the River of Blood, they stole not only the memories (normally up to everything involving the xerfilstyx before the draining) but the blood of their victims as well. After stealing blood, they could then spew forth an approximately 50 feet (15 meters) cone of boiling blood mixed with Styx water from their mouths. This attack (requiring the xerfilstyx to have drained blood within the last day and becoming more powerful the more blood they had absorbed) both scalded their foes and subjected all caught within it to the same power as their memory-stealing tails.
Xerfilstyxes were known to wield many spell-like abilities. At will, they could cast confusion, dispel magic, hold person, greater teleport, and wall of ice. Three times each time, they could use cone of cold, ice storm, and legend lore. Once per day, they could replicate insanity and power word blind. Once per day, they could attempt to summon another xerfilstyx, succeeding about half the time.
Although xerfilstyxes kept a sizable horde, which could include anything ranging from coins, coral, valuable jewelry, gemstones, potions, scrolls, and other magic items, they didn't invest their wealth into treasure but instead used it as bait. They might also take opportunistically take advantage of nearby events, such as memory echos creating visions in the Styx, in order to catch their prey. They fought from the safety of the Styx, always seeking to draw their foes close it, and after charging their foes could attempt to toss their opponents into the air (and into the river if they so desired). Often they observed those that had angered them from a distance, measuring their strength before taking wrathful vengeance.
Xerfilstyxes almost always started combat with a barrage of offensive powers, including ice storm and cone of cold. After doing so, they would often follow up with a blast of Styx water, having drained someone earlier to use it, followed by power word blind. However, xerfilstyxes enjoyed slaughtering up close and quickly moved in, charging at the closest foe before tossing them and scattering their adversaries with their fearsome auras. When they had only one foe against them, they would keep their victim still (using hold person if necessary) so that they could drain them. If a battle turned against them, they would spew a blast of Styx water to try and end the fight.
Despite their power, xerfilstyx had little in the way of cunning. They were arguably considered lesser or greater baatezu, purportedly sharing the same rank in the hierarchy as gelugons, yet few devils would seek out such a transformation. To become more powerful but less influential was often termed a "lateral demotion" and no devil given the option would choose such a form rather than, for example, a weaker soul-harvester. 
Because of their extreme unpredictability (especially to beings of law) most devils tended to avoid the insane xerfilstyxes. They in turn had little use for mortals or other devils. They were most likely to be found individually or in a "school", a rare event where a group gathered deep in the Styx to exchange information and memories. However, xerfilstyxes might also organize into "troops" with other devils comprised of one or two of their own number as well as a similar number of pit fiends, around 3-4 cornugons, and 3-6 hamatulas.
Though they primarily spoke Infernal, Celestial, Common, and Draconic, Xerfilstyxes knew a few bits and pieces from all tongues. Often they would switch between long-forgotten languages and a modern mode of speech in the middle of their sentences.
Xerfilstyxes were native to the Nine Hells, residing mostly on the banks of the Styx in Avernus where they guarded its shores and lurked near the water's edge to murder the unsuspecting. However, they were comfortable at any depth and could be found anywhere in the River Styx, even on other planes. If not at the river's bottom, they maintained nests in the Styx's inlets, sometimes just under the water, but fortunately they rarely had any kind of permanent lair since they frequently forgot where they had visited.
- Eric Cagle, Jesse Decker, James Jacobs, Erik Mona, Matthew Sernett, Chris Thomasson, and James Wyatt (April 2003). Fiend Folio. (Wizards of the Coast), pp. 59–61. ISBN 0-7869-2780-1.
- Robin D. Laws, Robert J. Schwalb (December 2006). Fiendish Codex II: Tyrants of the Nine Hells. Edited by Chris Thomasson, Gary Sarli, Penny Williams. (Wizards of the Coast), pp. 138–140. ISBN 978-0-7869-3940-4.
- Robin D. Laws, Robert J. Schwalb (December 2006). Fiendish Codex II: Tyrants of the Nine Hells. Edited by Chris Thomasson, Gary Sarli, Penny Williams. (Wizards of the Coast), p. 158. ISBN 978-0-7869-3940-4.
- Robin D. Laws, Robert J. Schwalb (December 2006). Fiendish Codex II: Tyrants of the Nine Hells. Edited by Chris Thomasson, Gary Sarli, Penny Williams. (Wizards of the Coast), p. 10. ISBN 978-0-7869-3940-4.
Least: Lemure • Nupperibo • Spinagon
Lesser: Abishai (Black • Blue • Green • Red • White) • Barbazu • Erinyes • Excruciarch • Ghargatula • Hamatula (Stony devil) • Kocrachon • Merregon • Osyluth • War devil • Xerfilstyx
Greater: Amnizu • Cornugon • Gelugon • Logokron • Narzugon • Orthon • Paeliryon • Pit fiend
Baatezu of unknown rank: Advespa • Dogai • Gulthir • Jerul
Araton • Burning devil • Fimbrul devil • Hellcat • Hellwasp • Imp (Bloodbag • Euphoric • Filth) • Kalabon • Kyton • Seared devil • Stitched devil • Succubus • Tar devil