I was re-reading all of the Story Hour. Yes, I'm that bored. Anyway, I came across a couple things that made me think more about this concept.
First, when I wrote my earlier post I was operating under the assumption that Agar didn't know about his bride until the Pit Fiend showed up. But, according to Agar:
Crouched in the stairwell, heart palpitating with repressed memories, Agar manages to focus an answer. "Yeah, pretty much so. I had managed to mostly forget about it. When I was a little kid, my parents arranged a marriage to a girl in another village. Later, when I started becoming a diviner, it was one of the first things I asked about. And?" Shudders rake his frame as insane memories start to creep in, but he manages to fight them down and continue. "And the divination was horrible. Horrible . I decided that getting married was something that I didn't want to do. It wasn't long after that when I started exploring the planes." He gulps. "I don't want to go back for the wedding."
So, from this, we can infer the following:
ITEM 1: Agar, a nice young Halfling, casts a divination that shows his future bride to be fiendish in origin.
First, which divination exactly was it? "Weal or woe"? Maybe something that showed him his future in-laws? Or did he make the same mistake Velendo did and ask something open-ended like "is she a devil?" Either way, there are plenty of ways he could have been led to think it was worse than it is. Or maybe half-fiend kids just look naturally ugly at first.
ITEM 2: Agar's parents don't know about Belle's true nature.
It's conceivable that they DO know and approve of her anyway, which'd be a nice twist, but if they did then certain other parts don't make much sense, so let's assume they don't know. Presumably, then, Agar never told them what he found out. Ergo, they've been operating under the assumption that this other family (which they must be close to by now) is just normal Halflings.
ITEM 3: Irony is a wonderful thing.
Out of fear of being married to a fiend, Agar runs away to become a planeshifter and alienist, a combination that in theory should leave him without many of the usual prejudices of form; after all, his motto seems to be "Anything is better with tentacles!"
He probably hasn't given it a second thought in the intervening years, still running blindly away, but if he saw a half-Erinyes half-Halfling, he probably wouldn't recoil any more. (Nolin sure wouldn't.) Especially if you house-rule the Half-Fiend template to take on some of the appearance of the parent; her wings would be feathered instead of leathery, so she'd look a lot like an angel except for the claws. Of course, this assumes you use the 3.5E templates; the 3E Half-Fiend only had a 50% chance of wings, and a wingless Halfling girl would probably be easier to cover with illusions.
ITEM 4: Erinyes have REALLY been nerfed in 3.5E.
They went from 11 spell-like abilities usable at will to 4. No Polymorph Self, Invisibility, Tongues, etc. On the other hand, they gained Greater Teleport and True Seeing, along with jumping from 6 to 9 HD.
This isn't just an idle point. The whole premise of the Erinyes is that it tempts mortals, which should be difficult to do if you clearly show devil features and don't speak the language. Kind of makes you wonder exactly how they can blend in, now doesn't it? It also makes the classic "Alix's stepmom was a chair" story impossible, so for internal consistency you might want to stay with the 3E version.
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Anyway, while I'm not sure you want to do this to the player, you could easily make this a "no way out" scenario. For example, after the PCs did the first divinations on this, they've assumed that the Halfling marriage contract has a standard "both must be Halflings" clause and that Belle is in violation of that.
What if Halfling society doesn't differentiate half-breeds? Unlike Humans, they can't just crossbreed with anything with two legs, so maybe it's never been a major issue to them. She is the child of a halfling, therefore she's a halfling, no matter who her mother was, which makes it legal. The subterfuge involved is dishonest but not illegal, which in many Halfling societies wouldn't even be an issue.
So, let's presume the marriage contract doesn't have any loopholes. How would the Defenders react? They can't really buy out the contract, especially if the bride's family has a serious motivation to see it go forward. For example, maybe her father wants her to take over the family business; she's smart enough (+4 INT for the template), but he's afraid that if he dies she'll lose any legal claim to the company under halfling law. So, he wants to see her married to a halfling before he dies, and there really isn't any acceptable substitute. Once her heritage becomes public, it's not like she'll be able to date normally, so this was the best option.
Maybe Hecabus is right and the pit fiend was really sent by BelGrande to cause problems. The divinations asked if it was sent by Belle's family, and the answer was "yes", but if BelGrande is a distant ancestor he's still family, so it's not a lie. And he has plenty of motivation. After all, his plan wasn't working out the way he had intended; it was originally going to be a simple item retreival from a rich Halfling family. Instead, he could potentially make enemies of a powerful group of living saints; so, he's cutting his losses by getting the Defenders to do his work for him. The first step is to get them worried.
The Pit Fiend shows up two months early in a clearly suspicious way, threatens the party, threatens Agar's parents, and in general gets the Defenders to start the divination chain. Once they realize there's no easy way out of the contract, they'll be looking to deal, and BelGrande wants the DoD to negotiate with him directly instead of trying to deal with Belle's family, since her family doesn't even know about the item he wants.
He'll ask for the item, and they'll give it against their better judgement; after all, it's not magical, so what's the big deal? What he won't say is that he doesn't actually have the authority to call off the wedding. Which leads to all kinds of problems, but once he has the item there isn't a reason NOT to go through with the wedding, not that Agar will think that.
I don't know, I just think it'd be really funny if after all was said and done, Agar was forced, kicking and screaming, to have the happiest day of his life. It'd be a nice counterpoint to the hopelessness of the whole White Kingdom situation, especially if you intend the campaign to draw to a close. "And they all lived happily ever after." All it'd cost is a trinket his parents keep on the mantel.
But, the Defenders have spent so much time defeating world-destroying plots that it may not occur to them that powerful enemies don't always have grand plans. They'll be looking for something bigger.
