Dealing with psionics means that you will also need to be familiar with the mechanics of the astral plane.
The astral plane is the spiritual counterpart of the material world. It is, literally speaking, "the spirit realm". It touches the material world at all points, being co-terminus with it, but on a different "wavelength".
The astral plane is the spiritual counterpart of the material world. It is, literally speaking, "the spirit realm". It touches the material world at all points, being co-terminus with it, but on a different "wavelength".
Creatures enter the astral plane for various reasons, most usually related to travel. Traveling astrally is the preferred method, and sometimes the only method, for reaching the planets, the stars, and far distant celestial bodies. [Functionally, these can be treated as equivalent to the "other planes" in the neo-Gygaxian worldview.]
Traveling Astrally - the Basics
People can enter the astral plane in one of two ways: 1) spirit projection, or 2) ethereal translation of their material body.
If done through method one, the projected spirit is connected to the material body by a silver cord. This is preferred for long-distance travel. The spirit body travels at essentially "the speed of thought" through the astral plane.
If done through method two, the physical body itself does the traveling, while in an ethereal state. This is much slower. The material body cannot travel any faster while ethereal than physical, and usually, is actually slower. The astral plane is more like moving through water. Because it is not solid, it is difficult to be "walked through".
Rather, while in an ethereal state, the astral plane is easiest to be "swum through" or "flown through". Thus, most creatures who travel regularly through the astral plane have wings (or develop something like the Thought Eater's web paws). [The Githyanki have perfected the art of sailing through the astral sea using the gossamer web of the Phase Spider for sails.]
Implications for Astral Combat
If done through method one, the spirit body is essentiall "naked". The only offensive combat type available would be psionic combat. The astral body is vulnerable to psionic and other attacks that directly effect the spirit body, such as the energy-drain of the undead or certain special magic weapons. The astral body is not physical at all, so it is immune to purely physical attacks.
If done through method two, equipment and magical items can be brought along into the ethereal state, and the ethereal body is vulnerable to attack. Thus, two ethereal bodies could engage in regular combat against each other on the astral plane, using both physical and magic effects. [Think of Frodo being stabbed with the ethereal sword and visible to his pursuers while wearing the One Ring.] As long as they are on the same ethereal "wavelength", they are solid objects relative to each other.
An ethereal body can change wavelength to attack a material body, in a fashion such as a Phase Spider. An ethereal body can SEE purely astral bodies, but cannot affect them without special magic items or some other "spirit attacking" ability. The Githyanki silver sword is such an object, a magic item that can sever the silver astral cord.
The base chance for an encounter on the astral plane is 1 in 20, checked three times, at the beginning, middle, and at the end of the journey.
Becoming Lost on the Astral Plane
The silver cord prevents the projected astral body from becoming lost. An ethereal body, however, can very easily get lost in the dark, gloomy, and windy astral plane. This is another reason that ethereal travel tends to be for very short distances only.
The most significant cause of becoming lost in an ethereal state involve an Ether Cyclone, which is a powerful storm version of the Psychic Wind. Basically, traveling in the astral plane is like swimming in the ocean, making the traveler subject to currents and whirlpool-like effects.
In Appendix C (pg. 181) of the 1e DMG, Gygax recommends a 5% (1 in 20) chance (for a short-hop trip) that an astral projection or ethereal journey will be disrupted by an astral current of some sort.
If Astrally Projected
1-12 --> slows travel, one additional check for random encounter
13-16 --> blown off course, lost for 2-20 days, extra check for encounter each day, then return to starting place
17-19 --> blown off course, arrives at an unintended destination
20 --> dangerous storm, save vs magic or have silver cord snapped, lost for 4-40 days, extra check for encounter each day, then return to starting place
If Ethereal traveling
1-10 --> blown off course, random direction, 12" per round of travel, one additional check for encounter
11-15 --> blown off course, arrives at unintended destination
16-18 --> lost for 5-60 days, extra check for encounter each day, then arrive at unintended destination
19-20 --> lost for 10-120 days, extra check for encounter each day, then arrive at unintended destination
What exactly is an "unintended location"?
Traveling astrally to the moon or a planet, an unintended location would be another planet within the solar system. Travel astrally to another solar system, an unintended location would mean a different solar system than the one intended.
Traveling ethereally, an unintended location would mean you land in something else's lair, such as a Mind Flayer, a Githyanki, an Aboleth, a demon, etc....
