How psions are different from core classes:
Psions can make for hardy adventurers, given time, but they start their careers relatively weak, as their special perception of reality causes them great stress and confusion. However, they catch up with a vengeance at later stages, and their Will saves become very powerful.
Psions have access to psionic powers, which are supernatural abilities initiated by the psion exercising their will on reality. While most classes have abilities that require you to try to roll high and reach or exceed a target DC, psions have to roll at or under their own ability scores. For a psion using a power, rolling low is good, just as with luck rolls in the core rules. The lower the better. Of course, when they perform more mundane actions, they try to roll high just like everyone else.
Many psion powers mention the die chain. For easy reference, the die chain is reproduced here:
Power Points: If you are using powers from older iterations of D&D, you have power points as per those systems. This build doesn’t concern itself with power points beyond that. The powers in this supplement are balanced by manifester level rather than power points, but you may find mind burn reminds you of the flavor a bit.
|Level||Attack||Crit Die/Table||Action Dice*||Powers||Ref||Fort||Will|
*Psions can use one power per turn, in the place of their highest action die. Additional action dice can only be used for other actions, such as attacking.
Hit die: d5
Powers: At first level, psionicsts have a number of psychic powers equal to their intelligence modifier plus a 3-sided die (minimum one power). At each level thereafter, they gain more powers equal to a die roll as indicated on table P1 (the intelligence modifier no longer figures into the number of powers gained, but it will affect their manifester levels [see below]). Psions may learn powers automatically when leveling, or they may have to gain them through more esoteric means, as per the whim of the DM.
Mental Alacrity: At the beginning of their adventuring career, psions are often distracted by the happenings in their own mind and in nearby minds and other phenomena they take notice of via the mental plane. They start with a lower action die than most other classes, but quickly gain more action dice as they level up. They have an initiative bonus equal to the number of action dice they currently have.
Manifester level: Psions manifest powers, and powers often reference the manifester level of the psion. A psion’s manifester level is their ability modifier plus their class level, but the minimum a manifester level will ever be is 1. Different powers are based on different abilities, so a psion may be at different levels when manifesting different powers. Some power results constantly change to match the manifester level of the being that manifested them.
Mind Burn: A Psion can burn ability points after they make a power check they are not satisfied with, unless it was a natural 20 roll. If they choose to burn, they lose as many points of ability as they needed to roll under that ability when they first made the power check roll. After the power starts to take effect, the psion’s manifester level will most likely become lower until they recover ability points through rest. You burn through the ability that you are rolling your power check against, but you always have the option to burn intelligence or personality instead.
Raising Manifester level through Mind Burn: You may also burn the appropriate ability points, to increase your manifester level.The amount of points burnt increases manifester level for one success result of a power by an equivalent amount.
Mental Recovery: Psion minds and their inner strength regenerates quickly.For the psion, intelligence and personality damage each heal at a rate of 1d5 per night of rest.
Power Check Roll: To see if you use a psionic power correctly, you need to roll a d20 and score at a designated ability score or below (the default ability is intelligence, but some powers list other abilities to check against). Failure to do so means the power does not work.
Roll 1: When rolling a power check, rolling a 1 is good, called a power score in common parlance, and something cool as dictated by the power's "Roll 1" description happens.
Roll 20: When a power check, rolling a 20 is bad, called a critical failure in common parlance, and something bad as dictated by the power’s “Roll 20” description happens. If something is allowing you to roll a smaller die, and you roll the highest result on that die, you need to roll a luck check or suffer bad effects as if you had rolled a 20. Sometimes rolling a 20 still causes the power to go off, albeit in a perverted form.