[Sendric]

From the Complete Adventurer:

Sneak Attack (Ex): A spellthief deals an extra 1d6 points of damage when flanking an opponent or at any time when the target would be denied its Dexterity bonus. This extra damage applies to ranged attacks only if the target is within 30 feet. It increases to 2d6 points at 5th level, 3d6 points at 9th level, 4d6 points at 13th level, and 5d6 points at 17th level. See the rogue class feature, page 50 of the Player’s Handbook. If a spellthief gets a sneak attack bonus from another source (such as rogue levels), the bonuses on damage stack.