An inordinate number of pages in both the Players Handbook and Dungeon Masters Guide are associated with the "optional" game mechanic known as psionics. It's unusual to find such abilities existing side-by-side with a magic system in a fantasy RPG, mostly because psionics are more of a science-fiction nod to supernatural abilities possessed by an individual. However, since Gygax enjoyed mixing genres, it's only natural that such an add-on to the game would be included. Psionics were first introduced near the end of the Original D&D era in Eldritch Wizardry, and then included in the Advanced D&D rules in 1978 as an official add-on. However, the rules themselves are poorly explained and, if properly played, would almost never belong to player characters in any event. However, the Monster Manual was liberally sprinkled with all sorts of monsters capable of utilizing psionics powers - from extraplanar creatures like demons, devils, couatl, and shedu, to the exclusively psionic thought eater, mind flayer, and intellect devourer. Most early campaigns dabbled in psionics in one form or another, but it seemed unbalancing in the long run. One of the characters in my second ever campaign in the Temple of Elemental Evil had psionics, a magic-user of questionably Lawful Good alignment. After battling with a psionic creature, however, he lost the ability to use his psionic abilities permanently (and I was surely glad of that). DRAGON Magazine offered the Psionicist NPC class, but no one ever used these as NPCs, and instead used them as new character classes to be exploited in the game. After 2nd Edition came along, no one paid much heed to the psionics rules and it all but disappeared after 3rd edition D&D (even though it was finally formalized into a character class all its own).
The AD&D rules on psionics were confusing and hard to work into a campaign. The rarity of such characters with psionics made any character with this ability immediately suspect unless the roll was performed in front of the group. No DM I ever spoke to allowed such abilities in their games - and after experiencing them, I too agreed. It was unfair that a PC could gain such abilities simply due to the luck of a random roll. However, Gygax also tried to limit these abilities to non-spellcasting characters or those without too many special abilities or multi-classing options (humans, dwarves, and halflings only). Also, the ability was only open to those with high scores in non-combat stats: Intelligence, Wisdom, and Charisma. Finally, psionic attack modes were downplayed so that they only worked on similarly endowed psionic creatures. The only attack that would work on non-psionic creatures was Psionic Blast (attack mode A) and it had limited uses. So, unless a demon had attack mode A, one rarely had to worry about psionic attacks being made upon them (most of the sciences and devotions had uses other than attacking).
The most confusing aspect of psionics was how to use them in combat if one DID face a psionically capable creature. There were a number of charts in the DMG relating to psionic combat, but it all seemed to exist outside the normal combat system of round structure and initiative. In fact, each psionic exchange took but 1 segment of time and it was suggested that "a good DM will usually just stop everything else until it is taken care of." That was another reason to not like psionic combats, since most of the other players would be sitting around until the one possessing psionics was finished having fun. Exclusionary parts of the game are frowned upon, especially when one character gets to do something while everyone else sits around waiting to find out what happened. And, let's face it, no more than one character per gaming group is ever going to be lucky enough to have psionics - it's just statistically rare, especially if you don't have the stats for it. Given the usual number of elves and half-elves in any game, it's just not likely that a character will be psionically gifted. Also, most DMs don't want to bother to learn a new combat system when they have enough to worry about in the game.
There are a lot of monsters in the game that have psionics and can use them to help or hurt the characters. A surprising number of "good" creatures in the game are psionic, and most extra-planar creatures are psionic as well. In fact, most creatures that can enter the ethereal plane or astral plane at will are psionic to some degree. Since the astral plane is considered a "plane of the mind," this makes sense (especially in the "psychic 70s" that focused on supernatural and psychic phenomena). I suppose that, like the addition of "kung-fu" monks and "mafia-style" assassins, it's no wonder that psychedelic powers were added to a game written in the 1970s.
Monsters with psionic powers from the Monster Manual include: brain moles, cerebral parasites**, couatls, most demons and devils, dwarves*, gray oozes*, halflings*, intellect devourers, ki-rin, liches*, men*, mind flayers, yellow mold*, shedu, su-monsters, thought eaters**, titans, and tritons*. Fiend Folio creatures with psionics include: algoids, Lolth, styx devil, elemental princes of evil, dark elves (?), enveloper, githyanki, githzerai, skeleton warriors*, most slaads, sussuri (?), and trillochs (?). Creatures marked with one asterisk are not inherently psionic, but indicate that there do exist creatures of these sorts with psionic abilities. In the cast of the thought eater and cerebral parasite (two asterisks), these creatures have no psionic powers but sustain themselves on draining the psionic powers of others. Creatures marked with a question mark indicate that it is unknown if these creatures have psionic capability. So it seems that Gygax made it very difficult to engage psionic creatures on their own terms (most listed are 5+ HD), but also made it hard for psionic characters to exist and fight off those seeking to capitalize on their windfall. Curiously, although devil and demon are given psionics, the Mezzodaemon and Nycadaemon were not. It seems that only high level daemons and daemon lords had access to psionics. Demodands and devas have no psionics except for the astral deva and solar.
The powers one gets for being a psionic character may be great or just so-so depending on random rolls. Also, determining your abilities for psionics takes a bit of advanced math. The average D&D character rolled using the 3d6 method will probably never get psionics because a 00 needs to be rolled on d% to get the abilities. Of course, if the character has Intelligence, Wisdom, and/or Charisma above 16, then this increases their chances of obtaining such powers. It's more likely to roll such high abilities using a different ability rolling method from the DMG or one of the alternate methods from Unearthed Arcana. Even with "4d6 drop the lowest" a character can get one or two scores in the range needed. However, statistics show that psionics are rare (as they should be) and thus will more than likely never enter a campaign at all. But what if they do? What does a DM have to prepare for in order to allow psionics to enter their new (or ongoing) campaign?
First off, a DM should be aware of psionics and how they are perceived and used in his campaign. I have always stated that monks are semi-psionic in nature; they perfect the body, mind, and soul and thus gain powers beyond the ability of normal humans. Their high ability requirement in Wisdom show that these powers are derived from insight and meditation, also useful to psionically endowed individuals. And, like monks, psionic characters should be exceedingly rare in a campaign given the requirements. Assume a typical character is rolled, and no excessively high scores are obtained (no higher than 15 in any score); the character is not very likely to be psionic, but the chance still exists (albeit very small, at 1%). The DM still needs to consider how psionics fits into the scheme of his or her campaign, because at some point it may happen that a character, replacement character, or even a henchman, could show up with psionic abilities. It's thus better to think this out ahead of time, than have to cram a new development into an evolved campaign that may not have such abilities considered. Although the number of psionic abilities is dependent on random rolls, the types are determined by choice. Having monks (a rare human-only class) be the keepers of psionic knowledge makes some sense here. Psionics could thus be better explained to the psionic individual if they journey to a remote monastery and speak to a Grandmaster in order to get better insight on their "condition." Such a monk may also be able to better explain the dangers of using psionic abilities, and some of the pitfalls to avoid. Having an in-game method of explaining the ins and outs of psionics is key to establishing these mental abilities in the game. PCs are typically rare among humans; this is not obvious given the number of NPCs who have character classes! According to the section on Henchmen in the DMG, "human and half-orc characters suitable for level advancement are found at a ratio of 1 in 100." So, going by that assumption, a population of 25,000 humans has 250 potential PCs, and of those, only 2.5 will have psionic abilities. Thus, the possibility of finding a psionic individual is 1 in 10,000. If a PC is rolled from that community, there are at best 1 or 2 other individuals with similar psionic potentials, be they PCs or NPCs. This is just going by straight statistics. It's possible that these individuals may not even be aware of their abilities since they have never been exposed to a catalyst or experience that would cause them to emerge. This is strictly from an in-game perspective; if I had the ability to read thoughts, or control minds you bet I would have kept that secret for as long as possible and used it to my advantage over the years!
But what about the rest of the campaign world? How would other races who are non-psionic deal with psionic creatures? Would they fear and despise them? How would magic-users approach them? With reverence and respect or revulsion and threats since psionics can easily do what it took magic-users years to perfect? One has to consider that most psionic characters would keep their powers secret, perhaps only identifying each other psychically (if they had the proper devotions to do so). Otherwise, they might masquerade as a dual or multi-class caster (harder to accomplish if dwarven or halfling). How would the demi-human races treat a psionic character? Would they be outcast as "dark dwarves" or witch-kin? And how would populations at large treat psionic entities they may encounter? Surely, magic is accepted in the AD&D multiverse as real and useful; but what would the populace think about mental powers that cannot be detected by magic spells or abilities that may enhance greatly the powers of the normal character classes? I see this being treated as if the characters where mutants in the Marvel Universe - people with inborn abilities that most humans could never have are treated with fear and mistrust. So it would be with psionic individuals who flaunt their powers. Worse, those who keep their powers secret are forced to use them only when they can keep them from others, breeding even more distrust and suspicion in a gaming group or community. Some players like that extra flavor in their campaigns, but those who roll the ability without even expecting it (or wanting it) might find it debilitating, especially if there are other players who wanted psionics and didn't qualify or blew their roll. The DM must take all this information into consideration before even allowing psionics into his game. A clear indication of whether psionics is even permitted should be explained before the game begins; some background explanation as to the ramifications should also be explained, should the characters want to roll. But a DM should only give away general information about how society treats such individuals, without spoiling the surprises that come from in-game knowledge gathering.
At some point, PCs will leave their mortal plane and adventure beyond the Astral Sea. They will find that psionics is much more prevalent in the Outer Planes among the servants of the gods than the Inner Planes or Material Plane. And it is that prevalence that defines the split between extraplanar creatures and mortal monsters. Psionics endow these creatures with more abilities than PCs will ever possess and leads them to being the truly powerful opponents that PCs need to face at higher levels. If the PCs have the same powers, they are elevated to the same level as these opponents and should be judged accordingly as being more of a threat (and hence, more of a target) by those creatures. At some point, the PCs will likely find a threat that causes the psionically endowed character more problems than the other PCs, turning his or her strength into a liability (consider that most greater demons have Psychic Crush which can kill with a single attack, but only works on psionically gifted targets).
