A creature from which a vampire routinely drinks blood usually adapts to the rapid and repeated blood loss, but a thirsty vampire can push too far, physically weakening her prey. The victim’s skin turns pale and her eyes become listless; others describe the victim as “wasting away” or “a walking corpse.” Few mortals realize that these descriptions are accurate, and the victim is, in fact, courting death.
After having its blood sucked out over and over again by a vampire, the repeatedly drained creature becomes used to the effects and builds a natural tolerance for energy drain. The repeatedly drained creature gains a +4 bonus on saving throws against energy drain attacks and the saving throw to overcome negative levels. It suffers no penalties from negative levels, though it still dies if its negative levels equal or exceed its Hit Dice.
Pathfinder Roleplaying Game Monster Codex © 2014, Paizo Inc.; Authors: Dennis Baker, Jesse Benner, Logan Bonner, Jason Bulmahn, Ross Byers, John Compton, Robert N. Emerson, Jonathan H. Keith, Dale C. McCoy, Jr., Mark Moreland, Tom Phillips, Stephen Radney-MacFarland, Sean K Reynolds, Thomas M. Reid, Patrick Renie, Mark Seifter, Tork Shaw, Neil Spicer, Owen K.C. Stephens, and Russ Taylor.