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
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A vision is simply narrated to the party or that player in a solo session
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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
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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
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Warn the party in advance either there will be a fake tpk or some "dream/vision shenanigans"
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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.
4.5 is the best imo. Let them know it's a vision, so they won't get legitimately mad, but don't tell them they're going to die until it happens.