Good analysis. A few possibilities:
1. It's about the children. Agar is the heir of a powerful wizard/cleric/whatever (if he's not on the family tree, perhaps Agar's great great grandmother was his illegitimate daughter but her illigitimacy was kept secret) who banished a group of devils from somewhere they want to be (probably not the prime since the pit fiend has shown up there but maybe she's just an agent) hundreds if not thousands of years ago. Only the heir of that whatever is able to undo the banishment. Agar's son or daughter would be that heir and through Agar's wife they hope to turn him to their side. Or alternatively, they just want to make sure that Agar actually begets an heir (sooner or later they'll turn someone in the line of descent but it doesn't have to be right now) so that
someone is able to break the curse.
1b. They could also have been bound to serve the whatever and his descendents in the long forgotten past and they hope that Agar's son/daughter will release them from that service.
1c. Maybe Agar isn't the whatever's heir yet. Maybe the devils' agents still have to kill off a half-dozen people who are ahead of Agar in the line of succession.
1d. Maybe it's not about the children. Maybe the devils will try an alternate/nontraditional wedding vow/arrangement that makes Agar's wife into his heir should he pass away, thus granting her the ability to release the devils.
1e. Maybe the devils specifically wanted to avoid option 1d because Agar's wife will be a devil after all and would make sure to extract a big price from the rest of them once she had the power to release them--that's why the devils want a nonstandard wedding arrangement that specifically stipulates that she is NOT Agar's heir. On the other hand, she wants the traditional arrangement where she is his heir. Or, alternately, she asks for a stipulation that he will release her if imprisoned or something like that and attempts to convince Agar to release only her and then to use him to negotiate a heavy price for the release of the other devils.
2. It's about the children. In this case, the devils want mortals that they can control. Perhaps some Greyhawk-like pact bars them from acting directly on the Prime but agents with human blood can do so.
2a. Maybe the deep magics of the world stipulate that those of legitimate mortal descent can operate on the prime (or do something else the devils want done) at will. Perhaps it also states that those of fiendish nature can be bound through magic if the true name given to them by their mother/father is known (this wouldn't work if there are lots of legitimate half-fiends in your world but would if there aren't). In this way, the devils would be able to compel compliance with their schemes on the prime through use of planar binding spells.
3. Maybe Agar's marriage and children are just the first step. There's an ancient prophecy that the endless blood war will cease when one legitimately born of both demon and devil leads one side to victory. The "devil" that Agar is supposed to marry is really a captured succubus. The devils then plan to marry their child to (or one of his legitimate descendants) to a devil and come up with the leader prophesied to bring them victory.
3a. Since this is supposed to be particular to Agar, maybe there's already devilish or demonic blood in his family.
3b. Or maybe it's a celestial descended fiend who will win the blood war and Agar's great granddaddy was an Aasimar.
4. Maybe it's about the marriage. There's some prophecy or other that Agar will destroy or conquer something (maybe a layer of hell), and that only his family will be spared. So, if the devils become family, they get to live.
4a. Maybe Agar's great ancestor didn't bind or banish the devils but created a weapon that would destroy them (or maybe all fiends or maybe all outsiders)--only those of his family would be spared (he was a half-fiend/tiefling/half celestial/aasimar).
4b. Maybe instead of sparing his family, it spares those under his family's protection. And in the marriage ceremony, he pledges his protection to his in laws (or is somehow construed to do so).
Anyway, food for thought.
Graf said:
Been thinking about Agar a bit. Nothing brilliant (or even interesting) really springs to mind.:
The demons have spent a lot of time and effort to get someone married to Agar. They were willing to wait…. Demons don’t do waiting as a matter of course. In fact I have difficulty seeing them wait unless they absolutely had to. So the demons have little power over the situation. And again it’s marriage, not control, not breeding but they are specifically having him –MARRY- a demon.
So to reverse engineer an answer:
*It’s specifically something about our favorite insane halfing arcanist. In the many years since his birth they could -easily- have found another high level alienist (or wizard) or halfing or what have you. So if they’re fulfilling a prophesy it’s linked very closely to either Agar’s family or his village.
*The demons don’t want to control agar, or rather, if they did there are many other easier ways of accomplishing this goal.
*It’s not about children or breeding something.
*Demons don’t marry themselves. Marrying is a mortal (or at least a prime) thing. So they are probably fulfilling some requirement related to the mortal world.
Marrying is basically joining together two families. It is possible that these demons are trying to join a mortal family to fulfill some sort of prophesy, or requirement.
The only thing that really seems like it melds together all of these threads (unless I’ve made a mistake) is some sort of prophesy or iron-clad-magically-enforced-rule. Anything else could have been circumvented more directly. And unless something pretty dangerous is preventing the direct route then I see a Pit Fiend as being pretty direct.
not really a cool answer but maybe this will set a spark in somebody's head.