What are known portals to the Abyss in Greyhawk?

Question

In the 2nd Edition Module WGR6, The City of Skulls, the players are to infiltrate Doraaka, Iuz' capital, to rescue a prisoner. They however cannot teleport there because, as the Master Elementalist Karzalin explains (p. 11):

We know that Iuz has a gate to the Abyss within his city, and the operation of that gate appears to distort planar structures within thirty miles of the area

So there is at least one active portal to the Abyss in Greyhawk during the time of the adventure, allowing demons to enter this world and spread their brand of chaos.

Are there other such enduring portals from the Abyss to Greyhawk documented anywhere in published modules, or official publications like Dragon magazine or novels? Is there maybe one in the great Rift Canyon? The main focus here is on 1st through 3rd Edition, where Greyhawk was a major supported setting, but any edition material would work.

Answer 1

List of published portals to the Abyss

Thanks to @Kirt for unearthing this thread where rasgon did a lot of the heavy lifting.

• Dragon's Maw, from the adventure Tammermauts Fate, originally appeared in Dungeon Magazine #106, dormant. (This answer)

• Shembai Oasis in The Bright Desert (from Rary the Traitor): "A Sulmish wizard once performed trans-planar experiments on this site, and the oasis' bad reputation is the result of a semi-active gate to the Abyss that still exists. Each night at sunset the gate produces 1-2 true, 1-4 greater, 2-8 lesser, or 2-12 least tanar'ri, who ravage the oasis and its vicinity, then vanish at dawn. The tanar'ri endeavor to drag any mortals they find back to the Abyss with them."

• Blackspear Chamber, Iuz‘ Palace in Dorakaa, as per the question, better defined in Iuz the Evil: "In the north wing of Iuz's palace, the Blackspear Chamber is Iuz's Gate to the Abyss. Fiends are summoned through it and upon their return to the Abyss, the powerful nexus strengthens them so that effects such as barkskins, stoneskins, and enhanced magic resistance benefit the fiends for ld100 days after their return to the lower planes."

• Fleichshriver from Iuz the Evil: "Fleichshriver has a gate to the Abyss which can only be utilized if an Archmage or High priestess is present."

• Onyxgate (Ivid the Undying): "Since the site radiates the presence of conjuration/summoning magic strongly, it is clear that some form of gate to the Abyss must lie beneath the surface... Worse still, there are persistent rumors that a dark magical artifact is responsible for the gate, Xaene's inscrutable hand is at work in having placed it here."

• The Null in the Sea of Dust (From the Ashes): "Ancient Suloise documents in Greyhawk mention a magical portal named The Null somewhere in the Sea of Dust-a place that is a universal gate to all known planes. It is said to be guarded by golems, summoned extraplanar creatures, stone sphinxes that test the wisdom of those who would dare travel using its magic, and more."

• Wakeportal (Dungeon #147): "This fist-sized mass of crystal, if embedded in the prow of a ship, permits the vessel to travel to the Abyssinian Ocean (in the Abyss) once a day. If already in the Abyss, it can be used to move the ship back to its plane of departure."

• Fane of Lolth, Underdark (Queen of Spiders): portal to the Demonweb Pits, Lolths plane in the Abyss

• The Causeway of Fiends (Ivid the Undying): "During the fullness of Celene, fiends of many kinds stalk the causeway. Tanar'ri and baatezu rend at each other, tearing each other apart. They gleefully attack anything foolish enough to approach within a half-mile or so of the causeway. The fiends appear to be bound to that distance, however, and cannot travel farther inland. Usually, but a handful of fiends will appear at full moon. Very rarely—perhaps once every 80 years or so—countless numbers of lemures, manes, dretches, and least fiends of all kinds will appear as great legions driven on by a few greater fiends." (Tanar'ri was the name TSR used for demons during the Satanic Panic years, baatezu was the name for devils, sothe causeway led to both the Nine Hells and the Abyss, although it might be one-way only)

Answer 2

Dagon's Maw, or the Pit of Hatred

Some SPOILERS follow for the Tammeraut's Fate adventure (in 3.5e version and also a part of the 5e Ghosts of Saltmarsh book).

The module Tammeraut's Fate from Dungeon Magazine #106, January 2004, features an underwater portal to the Abyss. The portal is somewhat protected, but creates some evil effects nearby. Currently it would be difficult for demons to reach most of the material plane from it, but it could be possible to remove the protections. From the module, bold emphasis mine:

Although this adventure is set in the World of Greyhawk on a small island off the coast of Nyrond in Relmor Bay, it can easily be adapted....

(Anna Meyer's Greyhawk maps show the adventure's town of Uskarn and Firewatch Island in Nyrond's Viscounty of Eventide, detailed map section 38.)

When the Tammeraut broke up and sank, the battered ship settled above an underwater chasm known as Dagon's Maw. This evil chasm once held a portal to an oceanic layer of the Abyss that had long ago been sealed and warded by the shamans of several tribes of sea elves native to Relmor Bay. The shattered wreckage of the Tammeraut landed atop this seal and damaged it.

At the bottom of the underwater cavern,

The funnel-shaped hollow to the north is 3 feet in diameter and descends 2 feet to a small vent. This opening is 1 inch in diameter .... The tiny opening itself descends for hundreds of feet without widening into the depths below. The forgotten architects of this opening left it as a pressure valve of sorts to allow the evil within to slowly seep out rather than accumulate and gain power over time. The elves further sealed the opening by enclosing it below a magically warded cover. The sinking Tammeraut broke these wards, and the slumbering evil is once again spreading.

The rift above the portal is not entirely unknown to surfacers:

A character who makes a successful bardic knowledge or Knowledge (geography) check (DC 30) also knows the location of the rift.

All this could be read as saying the actual portal no longer exists, but its side effects remain. But given the efforts made by the shamans intending to endure a long time, and the effects occurring since the magical seal was broken, I think it's easier to infer that the shamans were not able to actually close the portal, only restrict its access and effects, and the actual portal below the remaining physical barriers is still open.

The 5e source book Ghosts of Saltmarsh adapts Tammeraut's Fate as its Chapter 7, and still includes the portal to the Abyss. The details of the portal are very similar, except:

• Dagon's Maw is renamed to the Pit of Hatred.
• The exact location is not specified. Using the chapter within a longer Saltmarsh campaign suggests the rift might be in the Azure Sea closer to Keoland. On the other hand, Anna Meyer's maps put most of the Ghosts of Saltmarsh locations in or near Keoland, but leave Firewatch Island, and therefore the nearby rift, near Nyrond.
• The history and nature of the "rift to the Abyss" are not described in much detail at all. The chapter does say the PCs likely "discover the damaged magical seal that once kept the portal inside the Pit of Hatred closed". It also includes an "ancient warning" written in Elvish.