OK.
@AnotherGuy had some very good hooks, but those won't be enough, unless you plan on running a series of hooks, or can do something to engage them further.
I prefer to work backwards from motivations and outcomes. One of the big outcomes that the adventure doesn't spend time on is why the Devils/Baatezu are working against her. So, if you remember the FR Lore, AO (the overgod) forbids any of the gods from claiming the Prime Material as their domain (in the living area/Kingdom sense). This is for a number of reasons, but underlying all of them is that the Prime Material's inhabitants (those inconvenient mortals) are the ultimate source of all of the gods' power (with some exceptions) and mucking with that is a BIG NO-NO.
Tiamat, who depending on you beliefs about multiversal structures, may also be Takehesis - the evil goddess from Dragonlance - and not just a really really very very powerful dragon, is planning on physically incarnating on the Prime Material. This will let her boggart a very significant chunk of that worship power right off the bat, possibly get her a "purer" source of worship power, and basically give her the ultimate "win" in the deity competition.
She's succeeding in her plot because AO's ban is still in force, and frankly, most evil deities (most deities, really) aren't the subtle sorts, preferring to directly smite their problems. So enter the devil's ploys, and the Zhentarim.
Now, they'd really like to pull in the classic heroic do-gooders, as it's much less problematic, and works within the general rules. But making sure she doesn't win is a "MUCH* bigger concern. They (Tiamat's opponents) are prepared to sacrifice quite a bit, to make sure that doesn't happen. There's a lot they're prepared to offer up. Tiamat's Ritual Hoard is the least of it. Ditto for Divine/Infernal favors (even VERY large ones).
How about instructions on how to pervert some of that ritual power to make one or more of the party Immortal? Or Ascend? There's also the fact that they'll want a Hit Squad to remove Tiamat from the equation permanently - she can't be allowed to try this again, and others need to be dissuaded from copying her. How would they like to have her primary Hoard? Or any of the other implied rewards from becoming a God-Destroyer? (As opposed to God-Killers who "merely" kill a deities avatar.)
BTW, the god's aren't going to mess around with this. If the characters aren't attracted enough by "Kill Tiamat, take all her stuff" on their own, they'll make those offers directly and succinctly. They'll also make clear what the penalty is for refusing them - complete and utter obliteration of the characters and their souls.
At that point, if the players are still refusing, you won't really have a choice - the campaign is a non-starter, and all you can do is get out. And you should probably ask your players why they refused, because the rewards should have been pretty tempting.
But that's getting way, way, way, ahead of the battle. The Devil's Rewards side-quest is a solid place to start the offers.