Negative Energy Conduit (3.5e Trait)
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 Negative Energy Conduit
Your body seems to hum with suppressed power-that of the Negative Energy Plane. You can control this power, but it does have drawbacks...
Benefit: A character with this trait can active or deactivate a cracking aura of black electricity at will. This does 1d4+(Con mod) points of negative energy damage to anything adjacent to the character. Other than this, a character is healed by negative energy but injured by positive energy.
Drawback: The character takes an additional 1 point of damage per 5 points of holy damage. This also applies to the extra damage inflicted by a paladin's Smite Evil ability. Also, at the end of each day you must make a Fortitude save (DC 10+# of times aura is used) or gain a negative level until the end of the next day.
Special: There should probably, eventually, be some plot-related reason why a character has this trait. But there doesn't need to be one.
Roleplaying Ideas: Perhaps you have some sort of dark link to the Negative Energy Plane-or are marked by an evil god. This may cause those aware of your condition to become hostile, or fearful.