I have to admit, this turned more in my favor than I imagined with motivations, which I will explain here in a moment with added detail. The gist of what it is I have seen however, would be more than enough incentive and I think I have a few creative suggestions to build off what we already know so that we can bind them together a bit and make the entire process a bit streamlined.

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[i][b]1) You defeated a dangerous enemy, though they are still out there somewhere, gathering strength or forces. They hate or fear you now, at least to some degree, or at least you've got their attention. Who or what was it?[/b][/i]
The House of the Fiend is [i]the[/i] easiest adversary for all of us to face in plot alone. Every member has some reason to dislike them in concept from what I understand. I suppose the best way to add incentive to this without spoiling what I am suggesting is that fiends, actual ones, are immortal by themselves. Can you imagine what prices and lengths a warlock would go to ensure immortality... but only at the cost of their soul? There's ample room for not only misunderstanding, but legitimate evil to fight in the long run beyond a starting "contest of houses".

[b]2) During this event, you destroyed something important, or valuable to us. What was it?[/b]
A portal to the Feywild, while it works in part, works better as a Gate between planes in general. Given we are imaging fighting those of the fiendish house, this makes more sense. The implications also are higher stakes. The portal was destroyed, but it also severed and damaged others such as that of the Feywild and its general location - somewhere that was previously well hidden in the wilderness. Lorenthar would have dual motives [i]just[/i] to be present, in to defeat a common enemy, and to prevent a repeat of such an instance that caused harm to his faction's ideals.

[b]3) You paid a high cost for this victory. What was it?[/b]
[i]See part 2). Everyone lost something.[/i]

[b]4) You found signs that this incident was just the start of something bigger. What were the signs?[/b]
Not only would these warlocks be reasonably quite upset we severed their connection, they are now desperate. When you make an agreement with an unfathomably evil entity of tremendous power and you failed to carry out your end of the bargain - perhaps summoning them to the Material Plane or starting down that path - you tend to become a bit more... concerned. After all, death not only means you failed, but now you end up in one of the Hells for eternity, rather than living forever.

[b]5) All things considered, you impressed a powerful entity or organization, and they now fund your endeavors. Who was it?[/b]
There is a certain deity with an grim affinity who is not, to put it kindly, fond of anyone attempting to cheat death. Oh, she's also not particularly pleased with the fact that these individuals would also raise the dead into the unliving to enslave them or use them as potential other vessels for the unholy aspects. I think it was something along the lines of... the "Raven Queen", was it now?

[b]6) Why do you all trust each other? Was there ever any mistrust between members? Why do you remain together?[/b]
The sort of trust where you can remain assured [i]everyone[/i] involved has an interest. The stakes are too high - people binding evil spirits to themselves from the various levels of the evil planes to live forever and dominate the others with this power - not to. One of those scenarios wherein the group can expect every other member to act a certain way. You have nobility who are in this not only for the power to be gained, but the prestige of "fighting the good fight" and making an opposing house into a common enemy, to a woman who has devoted her life to a deity that [i]hates[/i] cheats, and then of course an Ally of Nature who has [i]no[/i] option but to destroy an enemy that wants to corrupt and pervert all life.

[b]15) Why do you care about the team? Do you even know all of their names? [/b]
[i]"We coexist at an unusual point in this time, wherein none of our associations alone can defeat this new enemy; to care nothing about a means to cripple and tear apart their hopes with assurance is to accept death, or worse. Failure means something to all of us. As strange as they might be individually, collectively the purpose is defined, just as the world intended. As for their names... a number of them use terms I find myself unfamiliar with or the meaning of. I have to wonder if they have multiple names."[/i]
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