Source: Book of Vile Darkness
The creature can make a spell-like ability particularly deadly to mortals.
Benefit: A mortalbane ability is a damaging spell-like ability that deals 2d6 points of additional damage when used against living non outsiders, but only half damage (rounded down) against outsiders, undead, and constructs. For example, if a mortalbane cone of cold from a gelugon would normally deal 45 points of damage, it actually deals 45 + 2d6 points of damage to a humanoid, but only 22 points of damage to a night hag. Creatures immune to cold - regardless of their type - still take no damage from a mortalbane cone of cold.
Mortalbane can be applied to each of a creature's spell-like abilities five times per day, though the feat does not allow the creature to exceed its normal usage limit for any ability. Thus, if a gelugon chooses to apply Mortalbane to its cone of cold ability, it can use a mortalbane cone of cold up to five times that day Thereafter, it could use its cone of cold ability again normally (since it can use cone of cold at will).
Special: This feat can be taken multiple times. Each time it is taken, the creature can apply it to each of its spell-like abilities five additional times per day.