Prerequisite: 3 ranks in Knowledge (planes), must worship a demon lord.
Benefit: Each demon lord requires a different obedience, but all obediences take only an hour to perform. Once the obedience has been performed, you gain the benefit of a resistance to some element or attack associated with your demon lord, as indicated in the “Obedience” entry for the demon lord.
If you have at least 12 Hit Dice, you also gain the first boon granted by your demon lord upon undertaking your obedience. If you have at least 16 Hit Dice, you also gain the demon lord’s second boon. If you have 20 Hit Dice or more, you also gain the demon lord’s third and final boon. Unless a specific duration or number of uses per day is listed, a boon’s effects are constant.
Demoniacs gain access to these boons at lower levels as a benefit of their prestige class. If you ever fail to perform a daily obedience, you lose all access to resistances and boons granted by this feat until you next perform the obedience.
Obedience: Perform a series of cuts upon the flesh while periodically auto-asphyxiating. Gain a +4 profane bonus on all effects linked to slashing damage.
What benefit exactly does this provide? I have a player who is arguing that taking the Demonic Obedience feat for the Nightripper adds +4 to damage with slashing. However, by my reading (of the Demonic Obedience feat) here the benefit is that it gives you a resistance to either an element or an attack type associated with that demon lord. Which means you would get a +4 bonus to resist something delivered by a slashing weapon. I'm not sure what those would be, but the other option seems far more ridiculous, considering you will also get the boons when you reach the appropriate hit dice amount.
Interpreting it as providing a +4 damage to slashing weapons makes it equal to having both Weapon Specialization and Greater Weapon Specialization, but in all slashing weapons. And only takes one feat. And Greater Weapon Spec is normally restricted only to fighters.
So what is it? What is it actually supposed to do?
+4 against effects delivered or caused by slashing damage. Ghoul paralysis, poison bites, etc.
So as I thought, it doesn't give any damage bonus.
The benefit is merely resistance of various types for different demon lords.