Here is my upcoming situation:
Soon (in the next session or two) the PCs will encounter a Hag (green) Coven. I know by changing a handful of spells, including something like wall of force, I can separate the PCs into two groups. Even at 9th level (which they should be by the time the encounter happens), a trio of hags (at CR 5) is a Deadly+ encounter. However, if I separate the party into two groups of two PCs each, this becomes much closer to a TPK while within the hags' lair.
Certain other spell options make this almost a certainty. One PC in particular is all about survival--so he might manage to escape death if the others fall.
The party is currently in the hags' domain and the power Grannie leader has had spies watching the PCs, so they are become more aware of which PC is going to be most problematic and in what way.
Now, their is an NPC werebear which is helping the party (the hags killed his mate years ago and he as been waiting for a chance to avenge her) but they still have about half a dozen encounters to go before the reach the lair. With attrition, I doubt they will be at full force when they arrive.
Of course, they are not bound by any means to do all this. They were sent to these ruins to investigate and want to help the werebear, and are hopeful with his help they can somehow rid the land of these vile creatures.