"One must not put a loaded rifle on the stage if no one is thinking of firing it."
- Anton Chekhov
The collected works of Stephen King to the contrary, writing a good ending is not a difficult task.
It's really not. It's easy, and in the context of heroic fantasy it's easier still. The key is this: that you are not merely crafting the ending to a story, but the ending to this story.
It's so easy that even Keep on the Shadowfell almost achieves it.
The principle of Chekhov's Gun suggests that every set-up must have a pay-off. Each dramatic concept introduced in the first two acts must have a part to play in the finale. Every decision the players make, every risk they take, every digression of conscience they choose to undertake will be rewarded or penalised in the story's climax.
Chekhov's Gun suggests that when the players rescue an ageing scholar, his advice will prove invaluable in planning the final assault. Chekov's Gun implies that when the players prevent the villain from acquiring an ancient mirror, they have bought themselves an advantage in the ultimate struggle.
Chekhov's Gun requires that the humble goblin that the players spared will repay the favour in the final hour. Chekhov's Gun calls on the villagers saved from the ravenous undead to buoy their rescuers' spirits when hope seems lost. Chekhov's Gun demands atonement for fallen stalwarts and vengeance for murdered innocents. Chekhov's Gun says that a holy sword bestowed by the ghost of a forgotten hero will always, always strike the decisive blow in the last clash of good and evil.
The final encounter of Keep on the Shadowfell doesn't do any of those things. Not one of the guns the module loads gets fired; instead, a bonus is bestowed on anyone who happens to be carrying one of the dragon statues they can only get from desecrating the altars to Bahamut located in the Skeletal Legion encounter.
The Shadow Rift has another problem, which is Kalarel. By now the DM is quite familiar with Kalarel's particular brand of incompetence, but for the players this is their first encounter with the final villain. It's hard to feel invested in his downfall - they can't hate him; they don't even know him.
Keep seems to know this is a problem, and spends a paragraph exhorting DMs to make him as hate-worthy as possible during this final encounter, apparently entirely by way of some dialogue that the module doesn't see fit to provide. One might be tempted to have him gloat about his evil plan, but that would require having a clear idea of what his evil plan actually is.
Besides, it's a bit late for Kalarel to gloat. This is the players' hero moment. This is their chance to kick ass and chew gum; it's the big musical finale. As DM, you've got approximately six rounds of combat to make your players feel like they well and truly deserve their victory, and any player who doesn't do something heroic is a player who's going to feel cheated when the dust settles.
So what does Keep get right in its last hurrah? The death of the villain.
As I mentioned when I was talking about the mechanics of this fight, when Kalarel hits bloodied HP he teleports to the glowing circle in front of the Shadow Rift. This raises his defences through the roof, but it also puts him only a couple of squares from the Rift itself.
This is the Rift. It's the portal to "Orcus' temple in the Shadowfell" that drove Sir Keegan mad and which Kalarel is attempting to re-open for frustratingly vague reasons. Kalarel has almost completed the re-opening and the portal is now semi-porous. Living beings passing through it will be killed instantly, but that doesn't stop an entity described as "The Thing In The Portal" from reaching its tentacles through to threaten players in the Rift's immediate vicinity.
Keep's been spending the bulk of its mechanical muscle teaching players about positioning through encounter after encounter. They've been pushing enemies down pits, over ledges, into traps and through holes in almost every significant struggle to date, and now at the closing of the day Keep deliberately positions Kalarel only two squares from a guaranteed auto-kill. It's just the right distance to be an achievable push, but just far enough to require your players to co-ordinate to pull it off.
When Kalarel hits the portal, or when players drop him to zero HP, the Thing in the Portal grabs the evil cultist in one of its tentacles and drags him screaming into the Shadowfell to meet his master. It's a great moment, because it's only possible through the combination of Kalarel's arrogance, the madness of his plan, and the prowess and acquired skills of the players. It's a thoroughly satisfying victory and it has the added benefit of having Kalarel conclusively defeated while leaving the door open for his return in later stories.
Wow them in the end, and you've got a hit. For all its wonkiness, for all its typographical errors and misfiring encounters, for all its dead ends and narrative buffoonery, Keep delivers a solid conclusion when the chips are down, and it's almost sad to realise that despite all the idiocy your players are going to walk away from the table considering that they've had a good time.
As DM, it's a bittersweet triumph. The module succeeds despite itself, and though Kalarel might be being tortured in the Shadowfell as we speak, the real villains of the piece are the module writers, who emerge from the debris unscathed and ready to produce more of the same under-developed tripe.