You delay the finale.
But you do NOT cancel the session. Or tell the players present that this is "filler".
Instead you plan some random cool
unrelated thing that the players who are coming can do. Something challenging & entertaining, but not likely to kill or permenantly deplete the characters attending.
Yes, they're approaching the final chamber. They can see it. Heck, maybe they can even see the final boss or the Mcguffin or whatever. Maybe initiative has even been rolled....
And right before they cross the threshold? A beam of golden light sweeps through hallway/chamber (Dex Save, DC = 1 higher than anyone present can make
(though you don't tell them that)), or there's a mighty horn blast, etc and only the PCs actually being played are shunted into some situation.
For example; Somebody, somewhere, sounds a Horn of Valhalla. Yes, the
book says it summons bezerkers.
But you're the DM. You could just as easily have it snag a few PCs, whatever their class.
And they appear with an unnatural bloodlust & the urge to attack/kill the horn blower. (oops, the horn wasn't matched with the guys class/lv.
)
The thing is now thy're stuck somewhere with no idea where/how for a while. And no way back save your whim. When the time limit runs out (IE the next time the full group can play) the characters are returned to the point where they were plucked from. "We now return you to your regularly scheduled finale...."
This general idea can also work via;
*Somebody monkeying with a Cubic Gate - they open a portal that just happens to be smack in front of the present players PCs. And closes over them in the blink of an eye.
* Q from Star Trek: The Next Generation. Q is a nearly omnipotent being, concerned only with his own whims. He's not at all fussed about the fact that these PCs were in the middle of something. Amuse Q & he'll send you home. Until then....
You can use this as a plausible way to dispose of the character from the player who quit. Catch him in the effect, not the normal NPC, play him as an NPC, and when the group gets sent home he's left stuck on the other side. (maybe Q took a liking to having him around or something)
And, if the player does rejoin, the character can return just as easily as he left with whatever extra lvs etc seem oppropriate.
Or if the player is gone for good? Then you've got a great future villain! "Remember me? You guys left me behind in/with _______. Now that I'm finally free....."
