I Had To See This Horrifying Old Dominatrix Clown Fiend So Now You Should Too.
Depicted above: paeliryons are 18 HD, huge-sized high-ranking devils. Ability-wise they aren't very impressive: they 'only' have 25 strength, and all other stats are worse than that, with most hovering around 20.
DR 15/good and silver is hard to overcome, acid and cold resistance 20 will come in handy, and fire and poison immunity have obvious uses. SR 32 is the double-edged sword it always is.
Offensively, the devil is more interesting. It gets a natural bite attack, but its main natural weapon are (deep breath) a pair of 30 ft-long extendible fingernails that damage charisma and get 17-20 3x crits. I'm not sure how relevant these will be, as even with all those goodies the damage is still somewhat disappointing (that and it still takes up your hands) but I'd be surprised if there's no weird niche for it.
In defiance of their description (which describes their smell as 'the stink of unwashed bodies and brimstone'), these devils have an Intoxicating Scent that applies the Mind Fog effect to others. It's free, it's useful, and your allies can be made immune to it.
Furthermore, paeliryons can Belittle their opponents, which stuns them for a while and leaves them shaken afterwards. It's a standard action to use, but it's a large stun AoE that your allies can be immune to (assuming they're cool with getting thoroughly mocked once per day), so if you don't feel like expending SLAs feel free to default to it.
Speaking of SLAs, they're plentiful! Notable ones include at-will Animate Dead, Antilife Shell, Blasphemy, Charm Person, Create Undead, Desecrate, Detect Magic, Greater Dispel Magic, Hold Monster, Greater Invisibility, Major Image, Mind Blank, Polymorph Self (upgraded into either Polymorph or Change Shape), Suggestion and Greater Teleport, as well as 3/day Blasphemy (yes, you read that right), Meteor Swarm, Permanent Image, Shadow Walk, and Symbol and 1/day Forbiddance, Implosion, Imprisonment, and Word of Recall. It's not the ultra-optimized spell list of a tier 1 caster, but it holds enough useful goodies that I don't think a paeliryon player will find themselves without some relevant magic.
Right now, I'm most comfortable assigning +1 LA: the paeliryon is a solid caster skewing towards usage of at-will spells rather than a few big encounter-enders (although it definitely gets encounter-enders as well), with a great chassis and a few useful side abilities. +2 or +0 are also possible, but seem less likely.
Imagine a bat-winged slug-centaur, but it's ripped as hell, carrying a sword, stark raving mad, and the size of an elephant. Whatever you're thinking of now: it's probably still not as bad as the real thing.
These monsters have 15 RHD, and are huge-sized. Their ability scores are considerably better than the paerilyon's: 29 strength, 25 constitution, and 20-12 in all other stats. They enjoy all the default devil abilities, in addition to cold immunity, DR 15/good and silver, fast healing 5, and SR 29. They're also immune to mind-affecting effects, piercing damage, and any harmful effects of the river Styx.
The fiends are quite offensively oriented. They have a wide range of natural attacks: two claws, a bite, a horn, and a tail. The latter can grab creatures and drain their blood, in addition to wiping their memory of the entire xerfilstyx encounter (which is certainly unique, albeit somewhat lacking in usefulness).
They also have the default devilish fear aura, the ability to trip enemies after charging them (which deals a little extra damage as well), and 10 virtual ranks in all knowledge skills (which exist for skill checks but not for meeting requirements or things like that). Additionally, they get a breath weapon, which has an impressive area and deals good damage, but needs to drain from a reservoir of drained blood (every max-strength 20d8 breath weapon takes 20 points of constitution, drained within the past day, to use). In addition to damage, it can remove all memories of the xerfilstyx encounter.
Finally, the SLAs. They're less extensive than on some other devils, but still broad. Most notable are at-will Blasphemy, Charm Person, Greater Invisibility, Polymorph (updated, of course), Telekinesis, and Greater Teleport, as well as 2/day Insanity and 1/day Foresight.
On top of all that, there's the Summon ability. Unlike most devils, there's no failure chance, it's usable twice (rather than once) per day, and it lets you summon from a wide variety of devils, up to two pit fiends. That's extremely impactful, and definitely worth considerably increasing the LA over.
The xerfilstyx is a weird and complicated monster, and I'm going to rate it +3 just to be on the safe side.