I was thinking of an idea of having one of the PCs experience by visions of dark future - sudden attack, party dead, all their beloved npcs killed, their home base destroyed. Visions would be sent by BBEG to tell that PC "if you won't join me, this is what will happen".
I was considering few options how to implement this in session, weighting ups and downs of each and thought to ask your opinion which is best
A vision is simply narrated to the party or that player in a solo session
Run a game where suddenly party is attacked by an overwhelming force and killed in combat, with that PC being last one standing, then reveal its just a vision they experienced and everyone is fine
As above but in secret talk with that pc's player beforehand and tell them this is going to happen and is not the first vision they had like this, but 2nd or 3rd
Warn the party in advance either there will be a fake tpk or some "dream/vision shenanigans"
Tell all players except the one whose pc will experience the vision and ask them to be my "co-conspirators" and help "sell" this moment by playing along.
I would avoid running any combat where the party is forced to lose. That would make any time spent on it feel like a waste. A short narrated vision would be fine, but if you want to make this a bigger encounter, you should give the party a goal that they can actually accomplish. I can think of a few ways you could do this:
The party could locate survivors among the dead to gain information about what happened.
Survey the wreckage, the party could use investigation skills to look at debris and piece together what happened. Think, crime scene investigation on a grand scale.
The BBEG may be showing a true vision of the future, but not a complete vision. Perhaps there is something that the BBEG is trying to hide, such as a clue to how to defeat them. For example, the party could use some kind of magic detection to see through an illususory water fountain and reveal a magic MacGuffin or a person who knows the BBEG's weakness.
I agree - player agency is important. If you do have them play through it; then even if the vision didn't happen or hasn't happened yet, let them get something from it - learn something they shouldn't, try things they couldn't (particularly if they realize it's a vision), etc. Heck, if they're enterprising enough, it might become a counter espionage opportunity! (Especially after the first time).