This is a rough draft, and I've really played with the class tables, so let me now what you think.
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:
d3–d4–d5–d6–d7–d8–d10–d12–d14–d16–d20–d24–d30
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 |
| 1 | +0 | 1d7/I | 1d16 | 1d3+INT MOD | +0 | +0 | +1 |
| 2 | +1 | 1d7/I | 1d20 | 1d3 | +1 | +1 | +2 |
| 3 | +1 | 1d8/I | 1d20 | 1d3 | +1 | +1 | +3 |
| 4 | +2 | 1d8/I | 1d20+1d12 | 1d4 | +2 | +1 | +4 |
| 5 | +2 | 1d8/II | 1d20+1d12+1d8 | 1d5 | +2 | +2 | +4 |
| 6 | +3 | 1d10/II | 1d20+1d14+1d8 | 1d6 | +2 | +2 | +5 |
| 7 | +3 | 1d10/II | 1d20+1d14+1d10 | 1d4 | +3 | +2 | +5 |
| 8 | +4 | 1d12/II | 1d20+1d16+1d12 | 1d3 | +3 | +3 | +6 |
| 9 | +4 | 1d12/III | 1d20+1d20+1d12 | 1d3 | +4 | +3 | +7 |
| 10 | +5 | 1d16/III | 1d20+1d20+1d14 | 1d10 | +4 | +3 | +8 |
*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
Abilities:
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.