I'd say only once per day, because it is specifically written in the half-fiend's entry.
Since the whole half-fiend smite thing refers to Paladin rules, I'd make it akin to the ability of the Anti-Paladin:
If the target of smite good is an outsider with the good subtype, a good-aligned dragon, or a good creature with levels of cleric or paladin, the bonus to damage on the first successful attack increases to 2 points of damage per level the antipaladin possesses.
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To confuse things a bit further: Please note that this is not the same as the Smite Good ability a creature with the fiendish template has (which has no entry of increased damage versus any special creature, and does not state overcoming any DR)(Edit: Deleted an erroneous sentence. Thanks Drejk!)
For consistency I'd go with having the Smite Good ability work like that of the Antipaladin from the Advanced Player's Guide. Only reason it isn't written that way is because the Bestiary came out before the APG.
Please note that the half fiend only gets 1/2 his HD as bonus damage (like a Pally of 1/2 his HD would)...
Were did you get that half-fiend uses 1/2 HD? I see that text says "the same level as the half-fiend's Hit Dice;"
Where did you get that half-fiend uses 1/2 HD? I see that text says "the same level as the half-fiend's Hit Dice;"
Oops. My bad. Somehow, I read 'half the fiend's HD'.
I guess I shouldn't post while having discussions at work.
Hello Folks!
I've got another rule question, but considering the difference, I made a new Thread.
Here comes the half-fiend with his "Smite Good"-Ability
Smite Good (Su): Once per day as a swift action the half-fiend can smite good as the smite evil ability of a paladin of the same level as the half-fiend's Hit Dice, except affecting a good target. The smite persists until the target is dead of the half-fiend rests.Source: http://paizo.com/pathfinderRPG/prd/monsters/halfFiend.html
So, does that mean, he can, even with many HD, only smite *once* per day? I would understand it so. So, second question: The paladin gets extra damage against Evil Outsiders, Evil Dragons and Undead. Does the half-fiend (or any other creature with this ability!) gets extra damage against anything?
And again, thanks for answers!
It is once per day, but he hits just as hard as paladin with the same number of levels as the monster has HD.