Regulation of Prime Portals

Sounds like the Guardians of the Gates.

^^Actually, the way I have it, even the fiends (other than the Obyriths) do that, since, as spiritual beings of this reality, the very presence of the Far Realm creatures sickens them (though not enough to hinder them). Plus, an Elder Evil from the Far Realm has the potential to mess up the status quo of the Outer Planes-- particularly when it comes to the physical and spiritual nature of one or more planes. Not even the Tanar'ri or Slaad would put up with some interloper waltzing in and distorting their territory, or worse, distorting the very definition of Evil or Chaos. As for the lawfully-aligned outsider races, you can bet they'd be dead-set against the very existence of such creatures in their Multiverse. Though I did have in mind that the powers and immortals of the Inner Planes would be especially opposed and hostile to Far Realm creatures and those tainted by them, since the Inner Planes represent the physical laws of the Multiverse much more than the Outer Planes do (which have more to do with mutable laws)

There is also some very specific stuff I had thought out regarding the incident that you might likewise be interested in:
Once the Elder Elves (more specifically, the task forces assigned with the Father llymic problem) are convinced of what Cryonax's objective is, they start trying to use him as an oracle, though Cryonax's help was limited. Among the things he told/answered were:
--He voices concern (which turns out to be mistaken at least for wizards and priests who don't focus on elementalism) that protective spells vs. cold won't offer any protection against Father Llymic's soul chill aura, since the entity doesn't follow the physical laws of the Multiverse
--He informed them of what was going on in the Outer and Inner Planes regarding the portals (the Elder Elves were under the erroneous assumption that the portals to the Inner and Outer planes were collapsing due to the breakdown of reality)
--He and his herald (Frigidora) are the ones who notice that Father Llymic gets stronger as more and more lifeforms succumb to brood fever (see Elder Evils for full details). He decides that the brood spawn must be destroyed, and advise the task force to slay any they find. However, the non-evil aligned members of the force are reluctant to do this and insist on seeking ways to cure their bretheren rather than kill them. Cryonax convinces them that, even if the condition can be cured without killing the host, the elves lack the resources to cure everyone AND have enough magic/etc. left over to fight Father Llymic and the other monstrosities.

Below are some bits and pieces that I've written out thus far for details of some of the timeline events (for my timeline topic)
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Several of the arcanists were adversely affected physically, many fatally. Blood curdling screams pierced the Vast Gate chamber as the flesh of some began to bubble, while in others it melted off like wax, condemning them to slow and agonizing deaths. Others still became horribly deformed, while in the most greusome cases, appendages such as arms, tentacles, even fanged stomachs sprang from victim's bodies and slaughtered their nearby horrified colleagues, often turning on their owners once no more victims remained within range.

The outsider and elemental races, naturally and viscerally revolted by matter and energy from the Far Realm, quickly became aware of these circumstances. In little more than a day, the entirety of the Great Wheel, Transitive, and Inner Planes are plunged into mayhem as the beings of each plane desperately scramble to smash every portal to Greyspace they can find, in order to contain the Far Realm's influence. Even Ptah and Celestian cut Greyspace off from the Deep Ethereal. One by one, the Elder Elves' various portals begin to fail, though some are merely sealed off, in the unlikely chance that the quasipowers, powers, and outsider races might need to intervene.
At the same time, the Vast Gate continued to expand, and by this point, the blot on reality began to destabilize space around Firestorm Peak. This caused innumerable smaller rifts to open into the Far Realm multiverse, including one into a highly inimical plane of cold and darkness. Another rift continually spewed out gallons of green slime from the Amoebic Sea, teeming with wriggling amoebae and larval wystes. The rift into the plane of cold darkness soon grew to monumental size, and horrible creatures begin to pour out-- entire armies of shivhads, and something far worse-- a crystalline being later dubbed "Father Llymic". At first, the Elder Elves mistakenly believed it had been killed after it tore a path of destruction into a verdant glade, only to wither away in the sunlight.

The struggle with Father Llymic attracted the attention of Cryonax, who sent several of his minions to deal with the situation. They failed dismally-- those not eradicated by the elves themselves fell victim to Father Llymic's corruption. Cryonax subsequently sent one of his aspects to the scene. When it likewise failed, Cryonax decided to take matters into his own hands, accessing the portal himself, accompanied by his herald and a small army of minions.
The portal took them (like those before them) to the base of Firestorm Peak, where they had an immediate run-in with one of the elven armies dealing with the aberrations. The general, after realizing that Cryonax was to Father Llymic (initially he made the normally fatal mistake of assuming that Cryonax had arrived to assist Father Llymic, along with a few other mistakes *namely, he referred to Cryonax as "that thing" and voiced the mistaken assumption that Frigidora was his "concubine general"*, petitioned him for advice. However, Father Llymic, Cryonax explained, was not a being of paraelemental ice-- or even of elemental air or water. Thus, Cryonax did not know how to destroy it. In fact, Cryonax noted, it didn't appear to be made of elemental earth or quasielemental mineral, either. Hearing this, the general's top advisor realized that the Far Realm creatures, in fact, are not composed of the basic elements at all, unlike all matter, life, and energy of their own multiverse.

As the two armies neared the spreading abyss of utter darkness, however, Cryonax's minions began to act strangely. Eventually, most either fled, become violently ill, or collapsed. It turned out that elemental and outsider races feel an innate disgust and uneasiness when near a Far Realm creature, and apparently it has the potential to make them physically sick. None of the monstrosities battled on the way to the gaping darkness seemed to have much effect on Cryonax's army, but the aura of Father Llymic was apparently too overwhelming. The advisor noticed that Cryonax and Frigidora appeared to be in great discomfort as well, though this seemed to have no effect on their abilities.

Perhaps. Although I don't think there were any prestige classes presented in it, you might be interested in Inner Circle's Violet Dawn campaign setting-- it's low magic and inspired heavily from Dark Sun (but with waaay more HP Lovecraft and less desert-- it has the exact same exotic feel of Dark Sun, though). For an ameteur college 3rd party setting it's pretty damn amazing. Sadly, they seem to have disappeared from the radar, but I believe their products are all still for sale, and I've actually encorporated some of their stuff into my writings on this site. (For the record, the only monster-related 3.5 book you need is Complete Book of Denizens. Aside from that, I would recommend the Avadnu Primer *which IIRC is free on their site*, the Broken Isles Lorebook (though some of it is redundant with the Complete Book of Denizens), and Player's Guide to the Broken Isles.

And, if you're interested, here is how I've incorporated the stuff:

--The Void is NOT part of The Farrealm; it is a demiplane created by Temulea (Temulea also created its inhabitants).
--Temulea was originally from the Far Realm *he crossed over waaay back in antiquity along with Ghaunadaur, Tharizdun, The Great Mother, and The Patient One. after Piscathes's bump created the Aboleth and left a festering wound in reality between the two multiverses. Back then, beings from the Far Realm multiverse sealed the rift.*, but was almost entirely naturalized to this multiverse. His alignment was originally chaotic neutral, but after he slew his son Krug, he went insane and became chaotic evil.
--The Luminas were actually my initial inspiration to make the Far Realm an entire Myriad Planes multiverse instead of just the Amoebic Sea. Though looking back at the Father Llymic story, this seems to be implied by the canon, as well. The Luminas are an epic-level celestial-like race with a Far Realm theme. They are extremely good-aligned, but in a very alien way (and in fact, their very concept of goodness is mildly alien to the D&D concept)
--In addition to the Violet Dawn canon, I have written up that half the reason Violetspace was cut off from the rest of the Multiverse was to prevent The Void and the Xxyth *and other voidspawn* from contaminating other parts of the Multiverse. (the canonical half was to limit The Void's interaction with Violetspace) (The voidspawn are Cthulhu-tastic monstrosities created by Temulea to undo his creation of Violetspace. They fill the niche of fiends in the Violet Dawn setting, just as the Luminas fill the niche of celestials)
--A colony Sulwynarii (not mentioned in the free primer, sadly-- they're basically neutral good elves with an affinity for crystals) became established on the Quasielemental Plane of Mineral shortly before the Time of Chaos hit Avadnu. Most of them are servants and/or cultists of Crystalle, the Quasielemental Prince of Neutral Mineral. Unlike most prime-indigenous flesh based races, the Sulwynarii are treated very well by Crystalle due to their natural inclination for cultivating crystals. Eventually I plan to work on encorporating the other major Avadnian races into Planescape, as well. The Violet Dawn books claim that the Sulwynarii are the ancestral form of modern elves, though I would say this clashes with the canon, and more likely they are either an offshoot race or another creation of the Leshay (racially they are a type of elf)

I think the original Keepers of the Cerulean Sign were the slave races that defeated the final Aboleth empire (if you go by my timeline, that would be the proto-reptile-like Chroch'hochaat and the proto-mammal-like Chroch'chikaat. They were wiped out by the Permian-Triassic extinction analogue inadvertently caused by Imix's rampage, however-- their extinction had nothing to do with the aboleth or any other aberrant race. In case anyone is confused regarding the flumphs and other primordial, indigenous aberrations, in my rules these are not affected by the Cerulean Sign. It likewise cannot be used to harm Inner or Outer indigenous aberrations on their own plane.)

Well, the Planarists do. That's Cirily the firre's group from Uncaged, known for their anti-prime bigotry. They don't want to protect the Prime from the planes; they want to protect the planes from the Prime.