I'm not trying to prove you wrong in this instance, but wouldn't the "Darkcloaked Spellthief"s

signature power be something like this:
Steal Spell (Free Action, Recharge 4+):
Trigger: The spellthief hits a creature granting combat advantage.
Target: Close burst 6 (the creature in the burst that was hit)
Effect: The spell thief can forgo its
sneak attack damage. If it does so, the spellthief may choose one unexpended at-will, encounter, or daily power with the arcane, divine, shadow, or primal keywords that is possessed by the creature. The target can no longer use that power until it has taken an extended rest. In addition, the spellthief can choose to either gain an action point, recharge one of its expended powers, or gain the stolen power. If it gains the stolen power, once before the end of the encounter, it may use the power as if it were the original target, including the target's ability score and other static modifiers, but using the spellthief's own temporary modifiers, such as combat advantage and power bonuses.