Ranes said:
Do you feel the need to justify what your extra-planar critters are doing on your PMP or do you happily shuffle elementals into your random encounter tables (or encounter generating mechanism of choice)?
Well, in my current homebrew campaign, Rhunaria, I posted a Rhunarian Primer on my website that explains what any reasonably-educated (through stories mostly) Rhunarian native would know about the world in general, as well as some things that only scholars, leaders, and adventurers would be likely to learn through their studies and interactions. My Rhunarian Primer describes how folk generally agree on the existence of at least 3 planes, those being Rhunaria (the material), the Ghostlands (the ethereal), and the Great Rift Between (or simply the Great Rift; the astral).
My Rhunarian cosmology is still mostly mysterious and not something that Rhunarian folk are familiar with, beyond these three simple distinctions, though I do have much of the general stuff figured out myself. The Ghostlands includes various things mixed in from other books and my own ideas, such as having an equivalent to the Deep Ethereal, and being chiefly like the Spirit World variant ethereal from Oriental Adventures and Manual of the Planes. It also has elements akin to the Plane of Faerie and Plane of Dreams. It is where spirits come from, and all souls reside on the Ghostlands superimposed over their physical bodies in Rhunaria, since the Ghostlands overlaps Rhunaria just as the Ethereal does the Material in standard D&D cosmology. But the Ghostlands extend beyond merely overlapping the physical world, they have depths and heights that don't even come close to touching the material plane of Rhunaria, places where the great spirits reside and where the spirits make war on one another, in some cases.
This is where many spirit creatures come to Rhunaria from, finding gateways between and traversing the Great Rift to arrive in Rhunaria with a physical body, since spirit creatures can't just spontaneously go from Ghostlands->Rhunaria without a physical body waiting for them in Rhunaria..... So they cross the Great Rift and in the process acquire a material body from that mysterious plane, and if they survive the journey, they enter Rhunaria to do what they will. Sometimes hordes of fiends come through to make trouble, but are often followed by celestial hosts trying to mitigate the damage and force the fiends back to where they belong. Elementals, Fey, and Outsiders are these spirit creatures that cross the Great Rift, though Dragons, Plants, and Undead are also spirit creatures of a sort native to the material plane Rhunaria, so to speak, in that their bodies are born, created, or animated in Rhunaria rather than coming from the Great Rift. Spirits are an important part of Rhunaria.
Do your players ever take you to task (if you're the DM) or do you take your DM to task (if you're a player) when you encounter an alien entity without the proper travel permit (ie it hasn't been summoned, there aren't any suspect, ten-foot-diameter discs of swirling otherness nearby and the critter hasn't just crackled into existence with an accompanying aroma of ground-level ozone)?
When I'm a player, I generally don't question the presence of such creatures in most cases, nor have players questioned me on such things in the past except for one time (where I gave them the short version of what I had been working on for posting to the website). The one time I was asked was in my current homebrew campaign, when the PCs fought a bunch of minor demons (one kind from the Monster Manual II, and another kind from Monsters of Faerun), and the sorcerer PC was wondeirng what that host of demons was doing in the area, after the party slew the demons. Generally, if there are extraplanar creatures about on the Prime Material or something, I figure that they came through (or were brought through) portals, Gates, or Plane Shifting casters elsewhere in the world, at some other time, such as a wizard long ago bringing forth some demons to do his bidding, then dying before he could force them back to their home plane, leaving them stranded for lack of any Gate/Plane Shift spells themselves. There are portals between the Ethereal, Astral, and other planes, for instance, though few if any there lead to the Prime Material (there are some though if I recall, from reading the Manual of the Planes). Of course such transportation methods are high-level generally, so there usually aren't tons of extraplanar critters roaming the Material Plane. And keep in mind that certain Outsiders and such actually do have personal Plane Shifting, Ethereal Jaunting, or similar abilities, allowing some to meander onto the Prime once in a while by chance or with a purpose.