Andomiel, that is wrong. Active skills are not noted in the monster icons, at least not in game (some sites have info about that using icons, but the game does not show those icons).
It simply means that their physical attacks have dark property, similar to a player buffed with holy weapon, except unholy. And yes, the effect is exactly what Krissa says: When you're being hit by a mob that "launches a dark attack," Unholy Resistance will act as a stacking PDef buff.
Well, I never noticed the dmg of those mobs since we do not have a buffer with unholy resistance yet in clan :( and since every mob with dark attack used to drain me I thought that was it, not active skills in general, just that it posseces dark attacks.
If u have unholy res buff the amount of damage is reduced, bit like resist wind, ive only noticed it on drain attacks tho as things tend not to live long enuff to cast there spells :)