It is not uncommon for characters to be able to summon creatures and even demons. The demonic spells start at 10th level and by 20th level you are summoning in some quite powerful demons.
So given the average life span of a demon why have none of them spent a mere decade or so researching a Summon Humanoid spell? A bit of hand waving and mumbling and ^poof^ there is a player character appearing before you. With a bit of opportune scrying and you summon the PC while taking their bath so they don’t have their +50 demon slaying sword and mithril armour of imperviousness with them either.
Surely what is good for the goose is good for the gander, as they say, so if PCs can summon demons then it works the other way around.
Imagine you are running a high level campaign, you start intermittently asking your PCs to make resistance rolls vs channeling (with a nod to BriH’s post yesterday). If they fail then the rest of the party see a rift open around the character and they disappear.
The summoning demon has assembled a squad of demonic minions waiting for the summoning to succeed and when it does then let battle commence. One rather unprepared PC vs an adequate number of demons of your choice.
You could even try and arrange it so that the PC is summoned during a battle. One which the PCs were probably going to win but was also a close run thing. You then yank one PC out of it. It disrupts the PCs strategic planning, changes the odds and puts one PC in a very dangerous situation.
You suddenly get another interesting option as well. Can the party rescue their friend from a different plane? Can they even find them?
For the, now solo, PC how do they cope without their party to fall back on?
I have lost count of the number of times that I have been on a quest for a specific item and when I finally get it I get raided by the forces of evil and they steal it from me. This is just a more impressive version of the same thing.
Spell research can be a wonderful thing, especially in the wrong hands!