last week's d&d session, my players completely derailed the entire campaign and i'm trying desperately to get it back on track. they were hell bent on getting into a city that was under lockdown, and they managed to get past 3 roadblocks i put in the way to prevent them from getting somewhere. they had an encounter with one of the BBEGs, didn't think he was actually a BBEG and wanted to attack him, but ultimately decided to leave THANK NEPTUNE. however, then they decided they were going to rally up a bunch of the town guard to go and fight this big group of evil mages. now they are legitimately going to try killing this dude and his entire group of mages and i can't think of really any way to prevent them from doing that besides things that would completely flip the campaign up on its head.
so my idea is going to be that him and a bunch of the other mages actually leave the castle, but not before doing some damage to the town and killing one of the old heroes (from my game) that they had captured. hopefully that dissuades them from pressing further. if they do, they have also already been warned that there are things in this castle that caused people to go mad... intellect devourers. they're easy to kill, but little dudes can literally just OHKO a party member.
in today's session, there could very easily be one or two character deaths. it's gonna' be real spicy. on one hand i feel bad because they did do a great job with getting around all these road blocks, and they had the assumption that this was balanced to their level despite me really trying to dance around the issue of dissuading them from progression further. on the other hand, i did try to dissuade them without outright telling them they'd lose... i had NPCs saying they thought the roadblocks were too strong and they'd never seen anything like them before, i had a magic lock with a DC of 28 that was only opened with a 5% chance on a magic item everyone forgot a guy had, and i even told them straight up "sometimes you might not have the intended way to do something and it's better to come back later" as well as "these people have magic strong enough to cast massive force fields, it's safe to say they might be very powerful." yet still, they persist.