In a world of magic and monsters even the most resilient minds can be damaged.  Sometimes it can happen as a child, witnessing his family's massacre at the hands of a horde of goblins.  Others are marked by staring once to often magically into the abyss.  No matter what the cause, the results are the same, the mind cracks and unlocks dormant power from within.  

There is a type of psychic scar for each for each discipline of psionics.  Sometimes the type of scar that manifests is in some way connected to the type of trauma the character suffered, and others seem to be completely random.  Scars can get worse, and consequently more powerful, over time, but noone can manifest more than one type of scar.  To have a psychic scar, a character must take the Least Psychic Scar of a chosen disipline at first level.  This represents a trauma that occured before the character became an adventurer.  That Scar can be strengthened by taking the appropriate feats at higher levels.

At the beginning the scar is purely mental, though when using its power a subtle glow can sometimes be seen somewhere on the persons body.  As it strengthens it actually becomes a physical scar that flashes with psychic power as it is used.
You should work with your DM to decide exactly how your character developed their scar.  Not only will this give you ideas about your character's personality, but it can also help your DM develop story hooks to personalize the campaign towards your group.

These rules aply to all psychic scars:

Least Psychic Scar must be taken at first level.  In addition to choosing a 0 lvl power, Least Phychic Scar gives your character the ability to gain psionic focus.  As long as your character remains psionically focused they can manifest their 0 lvl power at will.  

As the scar strengthens, your character gains a number of power points per day and the choice of a higher level power to manifest.  

Your character qualifes for any feat or prestige class that requires the ability to become psionically focused and manifest powers of their disipline of the highest level their scar grants.  They also qualify for anything that requires a number of power points less than or equal to the number granted by thier scar.

You cannot take this this feat if you already have power points or the ability to manifest powers or psi-like abilities from any other source.