Shadow Potence +X
All saves made by opponent vampires, lycans, skeletons, zombies, ghosts, and mummies take a -X penalty. (For ShadowSlayer)
All saves made by opponent vampires, lycans, skeletons, zombies, and humans take a -X penalty. (For NightHunter)
Remember that, once you add up all your save bonuses and penalties, the total can't be lower than -20. If this status would push the total below -20, then you only use -20.
Thanks to In Media Res.