The Dark Eldar almost always take prisoners during their raids. What happens to them is best not even contemplated. So, if the Dark Eldar win a close combat and force their opponents to fall back, roll a D6 for each enemy casualty in the assault. On a 4+, the model is taken as a prisoner. In addition, if the Dark Eldar advance and destroy their opponents by moving further than them, any enemy models removed are captured rather than killed. Place all captured models to one side. At the end of the mission each captured model is worth 1 victory point to the Dark Eldar player. In missions where victory points are not used prisoners have no effect on the outcome of the battle, but it's great for the Dark Eldar player to know just how many 'poor, unfortunate souls' he is going to take back.
Errata — Chapter Approved: Codex: Dark Eldar Q&A (v2.0): Necrons cannot be taken prisoner.