You draw forth a part of your own life force and (if you are not an undead) corrupt it into negative energy, which you can use to animate one corpse as a skeleton or zombie. Because the process of infusing the corpse with the negative energy is inefficient, you must draw forth twice as much of your life energy as what the undead would actually use. Therefore, you lose twice the number of hit points the undead creature would have when finished (so creating a normal Medium skeleton with 6 hit points costs you 12 hit points). Any skeleton or zombie created with this spell is treated as if it had been created with animate dead for the purpose of how many undead you can control. These hit points can be recovered normally (rest, magical healing, etc.)