I love that ending to KotSf. Feels really meaningful, even to lower level characters.
That said, I have also heard that it's an adventure that needs some tweaking - what did you do to "fix" it, if anything?
Spoilers for KotS ahoy!
Well- it is pretty flawed in a lot of ways. Basically nothing the pcs do (in the module as written) makes any difference to the way the story unfolds, except perhaps in that final encounter. It's kind of on rails. One of my main criticisms about it has to do with an encounter at an excavation site, where the bad guys are digging for... something, something that is never revealed but is supposed to be crucial to Kalarel's plans. Yet whether the pcs defeat the bad guys there or never even have the encounter in the first place, the rest of the module goes the same way. It's a pretty unfortunate aspect of the adventure. Another frequently leveled criticism has to do with a particular npc being too tough for the pcs, leading to many TPKs.
My run of it in 5e avoided both of those by not using the basic setup of the adventure-as-written and skipping everything but the dungeon under the keep itself.
As written, the actual dungeon is only about 60-70% of the adventure. There are preliminary threads that lead the pcs to it after they first have some other encounters. The forces led by Kalarel are threatening a nearby town, there's a kobold lair, etc.
I used none of that.
In my game, the pcs were traveling with a halfling baker named Hembletopp, and as they were setting up camp one evening, his nostrils flared and he declared that he smelled a precious and amazing ingredient being cooked- a mushroom called a fat lurker. The pcs found a goblin encampment, defeated them, and interrogated one of them, who told them that the lurkers came from the dungeon below a ruined keep. So in my game, the pcs' whole actual quest was to find these mushrooms for Hembletopp.
As they went deeper in and confronted more of Kalarel's minions, they gradually learned about him. The deeper they went, the more fat lurkers they found. So their whole enmity with him grew over time as they encountered more and fouler minions of his.
When they finally got to the final encounter, they had already effectively stopped his scheme. That portal to Orcus wasn't going to open without another sacrifice. The entrance to the room they fought Kalarel in was via an open hole in the floor of the room above, with bloody chains descending into it. When they found the hole, they could hear someone- Kalarel- weeping below. They called out, maybe thinking he was a prisoner initially, but he immediately stopped crying and invited them to come down, claiming that it was "glorious" and "magnificent" down there. One of the pcs- a hadozee- glided down and the fight started.
So basically, I ditched the module's first third and all the plot elements that weren't in the dungeon itself, and completely changed the reason and method of the pcs finding the keep in the first place.