You animate a corpse as per ''[[SRD:Animate Dead|animate dead]]'', transforming it into a skeleton or zombie. However, it only lasts as long as you concentrate. On the other hand, it costs nothing in material components and the skeleton or zombie uses your caster level as its base attack bonus. When you cease concentration, it crumples but may be re-animated with another castingif it was not destroyed (brought to 0 hp). It retains any previous damage it had upon reanimation. It does not add to your total pool of undead you can control at one time.