Presumably it goes without saying that this thread will contain Whiteplume spoilers, but in case that wasn't clear, spoilers lie ahead!
I ran Whiteplume recently as a self-contained adventure (took them 8 or 9 sessions to get through --- my group is extremely thorough) with a group that was usually 6 players. Level 8 as recommended, but I let them do some shopping for magic items ahead of time, making skill checks, etc. and rolling on tables based on their checks, etc. Point is, they got some good stuff and so they were certainly ahead of the normal level 8 power curve, even individually; add to that the fact that there were six of them, and clearly the encounters needed beefing up.
Like you, I thought the climax ought to be a showdown with Keraptis. Wound up being a super fun encounter that really went down to the wire between victory and TPK, but ultimately resulting in their victory; probably the most dramatic fight I've run.
First a little context:
They'd gotten Whelm and Wave, and meticulously fought through every last sea creature in the aquarium (I told you they were thorough), heaing for Blackrazor... Obviously an Oni was not going to be a challenge, so I made Qesnef a 13th level Valor Bard using a custom Death Slaad race. As written, he was being held prisoner, but I had him initially being controlled by Blackrazor, but making saves when he took damage to break free. Wound up rolling really high pretty early on, and so he starts to bolt with Blackrazor... except where's he going to go? Naturally he is stopped on the way out by the efreeti who drag him back toward his prison, and so the PCs wind up facing the efreeti and Qesnef in a three-way fight in the now-drained terrace room. That was also fun.
I wasn't going to have them just face more efreeti, so I decided the end fight was going to be Keraptis. The only place they hadn't been yet was the kayak river loop, so I decided that Keraptis was going to create a portal in the river to his demiplane, which, naturally, was in the form of a library. Played the creepy Willy Wonka river boat scene for them on the way in.
Their kayaks were ejected into the middle of this library, with several big bookshelves on the ground floor, and a 30' high catwalk around the edge, with some of those movable library ladders to get up and down. When they entered, the real Keraptis were invisible up top, but a programmed illusion of him was there to greet them, behind the big desk in the center.
He revealed that the name Keraptis wasn't a person but a designation that had been passed down over the centuries (think Dread Pirate Roberts), and that, as he was getting ready to retire, he wanted to designate one of the PCs as his successor, who would not only inherit ownership of White Plume Mountain, but would also gain all of Keraptis's magical knowledge. All they had to do was fight each other to the death for it; last one standing got everything.
I had him explain that they would have to decide individually (telepathically to him) whether to accept his offer. If they all refused, he would simply kill them himself; but if the decision was not unanimous, they would still have to kill each other, but he would put his thumb on the scales in favor of anyone who entered the fight willingly. Classic Prisoner's Dilemma, in other words.
So then I had the players hide a d6 in their hands, choosing a side to face up, representing their character's decision. I came around and looked at each person's die.
They stuck together; it was them vs. Keraptis.
To show that he had no hard feelings and that it wasn't personal, he had some automatons appear to offer them mysterious purple beverages, one for each person on a tray. I had a table of six effects, all including "the benefits of a short rest" and another effect, two positive, two mixed, and two negative. (The best one was an instant level-up and the benefits of a long rest, one was you regained some spell slots; the mixed ones were a wild magic surge, or gaining a version of a Ghast's Stench feature for ten minutes; the bad ones were being hit by the Blindness spell, or being intoxicated such that your attacks had a one in six chance of hitting yourself instead)
Most of them declined, or poured the drinks on the floor; two drank them and got the "hitting yourself" effect and the Ghast's stench effect.
Then it was combat time.
Keraptis rolled high on initiative, and immediately casts Wish to create a Simulacrum. The real Keraptis and his naked simulacrum appear on the catwalk; meanwhile the Programmed Illusion is still sitting behind the desk.
The party tries to get up to him, and naturally just as the melee characters are closing in (which takes a couple rounds, he uses a legendary action to Thunder Step to the other side of the library. Meanwhile a fireball has ignited the pile of kayaks and injured those who stayed below.
On his next turn he grabs on to the railing of the catwalk and casts Reverse Gravity. He and the characters who made their save are now hanging from the railing, while those who failed are on the ceiling having taken falling damage.
And so on and so forth.
They manage to dispatch with the Simulacrum reasonably quickly (I did not give the simulacrum legendary actions!), but the real Keraptis wears them down, several of them going down multiple times only to be brought back up by the life cleric... But the casters use up most of their spell slots, and everyone is very low on HP... but ultimately Keraptis has to burn through a ton of slots casting Shield, and is down to his last spell slot (which is 5th level). He is planning one last Steel Wind Strike, but is cut down by the Whelm-wielding dwarf paladin just before his turn comes up.
It was awesome.
