Derro are afflicted by a form of racial madness, which most often manifests as delusions of grandeur coupled with an overpowering urge to inflict torment on other creatures. Derro are capable of holding their murderous impulses in check for short periods of time in order to cooperate with creatures of other races, but such arrangements rarely last more than a few weeks. Of course, no derro is capable of recognizing that he is out of his mind.

Derro are stealthy and bloodthirsty. They like to carefully arrange cruel traps and deadly ambushes, and strike savagely from hiding. They delight in taking captives who can be tortured to death later, and favor traps and poisons that disable without killing.
Madness (Ex): Derro use their Charisma modifier on Will saves instead of their Wisdom modifier, and have immunity to confusion and insanity effects. A derro cannot be restored to sanity by any means short of a miracle or wish spell.
*The racial madness of the derro provides a +6 bonus to their Charisma scores and a ?6 penalty to their Wisdom scores. A derro restored to sanity gains 6 points of Wisdom and loses 6 points of Charisma.
Poison Use (Ex): Derro typically carry 2d4 doses of greenblood oil or Medium monstrous spider venom, applying it to their crossbow bolts. Derro are not at risk of poisoning themselves when handling poison.
Sneak Attack (Ex): Any time a derro?s opponent is denied his Dexterity bonus to AC, or if a derro flanks his opponent, he deals an extra 1d6 points of damage. This ability is just like the rogue?s sneak attack and subject to the same limitations.
Spell-Like Abilities: At will?darkness, ghost sound; 1/day? daze (DC 13), sound burst (DC 15). Caster level 3rd. The save DCs are Charisma-based.
Vulnerability to Sunlight (Ex): A derro takes 1 point of Consitution damage for every hour it is exposed to sunlight, and it dies if its Consitution score reaches 0. Lost Consitution points are recovered at the rate of 1 per every 24-hour period spent underground or otherwise sheltered from the sun.
Skills: Derro have a +4 racial bonus on Hide and Move Silently checks.

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System Reference Document -> List of Creatures (SRD)