Except that isn't true exactly.
Yes, Irian is a plane of positive energy but also order, hope and light. But what happens if you go there? There are things to do and people to meet. There are Lantern Archons, Ravid, Lumi, and Xag-ya living there. You can go to the Amaranthine City in the Zone and speak to sages, healers, and celestial beings, or heal a terrible wound in the Waters of Life. It is a place people go to, come from, and can exist in.
The Positive Energy plane? It is a mass of energy that explodes you if you are in it for more than a few minutes. It has no features, no locations, no inhabitants.
Well, if you are looking for exactly true . . .
In the
1e Manual of the Planes they call out Xag-Ya and Xeg-Yi as inhabitants of the energy planes and Trillochs as immigrants to the negative material plane.
Apparently there are places and structures in the 1e negative material plane. "The Negative Material plane is eternally dark,
its structures and towers made up of physical, solid blackness."
They also talk about a couple of things on the energy plane border areas as places to go.
"Towers: Ringing the Positive Material plane is a scattered group of great towers, massive structures of the heaviest elemental material available for that quasi-plane (blue flame, lead, solidified clouds, or ice). These towers extend into the Positive Material plane on thin peninsulas of quasi-elemental material. Some of these peninsulas so thin that a halfling could touch the Positive Energy plane on both sides. These towers are normally abandoned, but occasionally powerful creatures dwell within—high-level wizards, druids researching the nature of the Positive Material plane, or exiled Powers from the lower outer planes. The origin of these towers is as yet unknown, but it is noted that the intrusions of the Positive Material plane do not overcome the elemental peninsulas or the area around them."
"Citadels: Travelers in the planes near the Negative Material plane often report lights and activities within that plane very close to the quasi-elemental shores.
These mysterious citadels are apparently domed and complete fortresses or cities. They are tied to the quasi-plane by a thin rope of elemental material, so as to prevent their being lost in the negative energies beyond.
The inhabitants of these citadels vary according to the tale: great monsters, quasi-elementals on the verge of becoming archomentals, fell necromancers, liches, and lords of the undead further strengthening the ties between such creatures and the Negative Material plane. In truth, these citadels must be discovered by the travelers and are left as special encounters for the DM."
In the
3e Manual of the Planes for the positive energy plane they call out ravids and xag-ya. "Some outsiders make the Positive Energy Plane their home. The best-known of these are the ravids, which tend to dwell in the quieter areas of the plane, but the energon known as the xag-ya is also common, even in the deepest heart of the plane."
So 3.5 Eberron's Irian adds Lumi and Lantern Archons? I have never really delved into a lot of the specifics of the Eberron planar cosmology beyond knowing there are 12+1 planes and they move in and out of conjunctions.
The 3e MotP also talks about some features of the positive material plane
"Edge Zones
The edge zones are a reference for quiet areas on the plane, like islands or shores on the seething hotbed of energy. These regions have the minor positive-dominant trait and are dotted with bits of flotsam from other
planes, including floating citadels, bits of tattered astral haze, and shards of other planes. The more solid pieces of the edge zones are used as outposts by creatures powerful enough to weather the changing nature of the plane itself. Such strongholds must be well protected, because tides of more intense positive energy could sweep over the edge zone at any time.
The Hospice
A particular location within an edge zone, the Hospice is a floating citadel with a large outcropping of rock raised as a shield against the more lethal energies of the plane. The Hospice and the area within 300 feet of it have the minor positive-dominant trait, though the structure sometimes has to relocate in order for this phenomenon to be maintained.
The Hospice is home to a small community of holy knights and healers dedicated to the healing arts. The order is legendary for taking badly wounded individuals and restoring them to health, and the members know spells and procedures that allow the healing of otherwise incurable ailments. Their ability to treat diseases on this plane is limited by the nature of the plane itself, but even then the Hospice community may know of effective treatments that do not involve positive energy.
The Hospice is protected by a number of golems in addition to its humanoid staff. Good-aligned individuals from a dozen planes staff the Hospice. While they would not turn away an evil individual, they do keep less trustworthy patients in locked wards.
Imprisoning Cells
Particularly powerful individuals can be effectively imprisoned by dumping their physical forms or their spirit-bonded souls into a prison protected from positive energy and sent onto the Positive Energy Plane. While not a long-term solution (such prisons are invariably opened by some curious traveler or swept through a vortex onto another plane), these imprisoning cells keep items and individuals away from the rest of the planes for decades or even generations."
3e has some features and adventuring hooks for the negative plane as well:
"Doldrums
Certain regions on the Negative Energy Plane are less deadly than others, reducing the negative-dominant trait from major to minor or even removing it entirely. These areas, called the doldrums, are relatively static on the plane, so towers, cities, and other structures can be built at their locations.
The perils of such places are twofold. The most obvious threat is the hostile life (and unlife) in the area. A second threat is that the borders of a doldrums area may fail and the deadly tides of negative energy once again wash over the region. Necromancers in particular favor the doldrums for their lairs.
Death Heart
The best-known location within one of the major doldrums, Death Heart is an entire spired city constructed within a hollow metal sphere one mile in diameter that drifted in from some long-dead alternate Material Plane. While the exterior of the sphere has a minor negative-dominant trait, its interior is free of the baneful negative energy of the plane. That protection failed to save the city’s inhabitants.
The city was founded as an experimental utopian community. Originally called the Heart of the Void, it was designed by its mysterious masters to be a place untainted by other beings and schools of thought. In reality, it was quickly overrun by the undead, who feasted on the flesh and souls of the students within. Now its towers and plazas are empty except for the undead invaders. Here may be found all varieties of undead, including not only energy-draining creatures such as wraiths, wights, and spectres, but more mundane skeletons, zombies, and mummies. Several liches and powerful vampires claim this sphere as their home.
Rumors carried by the mercanes state that usually the various evil factions within the Heart of the Void are engaged in perpetual war with one another. But now a particularly dangerous individual, a vampiric minotaur, has brokered peace among the factions and has encouraged further research into the nature of the city and the plane itself. The mercanes believe that the vampiric minotaur’s eventual goal is to steer the city to another plane and use his undead minions to wreak havoc there.
Castles Perilous
Given the large number of undead creatures in residence on the Negative Energy Plane, there is less tendency to use the Negative Energy Plane as a repository for evil prisoners and dangerous items. However, good-aligned prisoners and benevolent items are often imprisoned here, usually in towers of pitted iron with sealed gates. These prisons are well trapped and often heavily guarded.
Some hold celestials or good-aligned artifacts not easily destroyed, while others contain paladins in stasis and other items and individuals baneful to the undead. The presence of these castles perilous is often a reason that Material Plane travelers (especially those of good alignment) come to this plane."
These are very different concepts.
And yes, if memory serves me going deep enough into Mabar can kill someone, but it is the realm of death and decay. That is like saying walking to close to a nuclear reactor can kill you. However, on the edges of Mabar? Still inhabitants like Death Giants, Gloom Golems, yugoloths, ect. Still locations to visit. It can kill you if you aren't careful and smart, but it isn't the "within 1 minute of arrival, you are dead" that we see from the Negative Energy plane. And, again, it isn't JUST negative energy, but also gloom, darkness, decay, entropy.
Irian and Mabar and the Shadowfell and such seem like definite attempts to make these types of cosmological features more suitable for adventures than the great wheel energy plane set up, but the older Manual of the Planes great wheel in-depth descriptions also have features for them and just like the later Mabar, the borders are more suitable than deep into the plane. So it is more like a step further on the same continuum than a quantum jump of nothing to something. 1e PH and DMG and MM mention energy material planes and there is some discussion of their relationship to spells and some monsters, but then the Manual of Planes fleshes them out a bit with some features to explore or use as planar narrative elements.
