The Big Bad makes a decisive move, advancing towards their ultimate probably-apocalyptic goal. Acquiring a magic item, sowing chaos, raising the first of an army of monsters, that kinda thing.
This is about as bad as things get. But also a glimmer of hope.
Because now, after hints and threats and possibly some violence, the Big Bad's plan is revealed. And that also reveals how to stop it.
Because if they're doing that it means they're planning to do that so therefore if we do this...
It's impractical, crazy, dangerous, probably all of the above and then some. But. It. Just. Might. Work.
Oh, and the Cast need a few bits and pieces. And so does the Big Bad. And some of them might be the same things.
And even if they aren't, this is probably the Big Bad's last opportunity to taunt from a position of safety. Use it well.
This could be a series of arc story scenes throughout preceding episodes, or all put into the third-last episode of the season, depending on how much weight you want to put on the Big Bad. A whole season could be made up of arc episodes, but that can get pretty wearing.
Take a look at previous episodes and see which plot threads could become Chekov's Guns. That troll hammer comes in handy weeks after being left behind in an apparent MOTW episode.
Any allies the Cast can call on in their hour of need? Any of the Big Bad's camp likely to betray the cause?
Example: On The Rocks
The Siren plans to destroy the city, draining the life from her victims and extending her lifespan by a thousand years. For this she needs the remaining pieces of the Stone of Fire, so that she can call up the storm she needs. And she's already arranged a captive audience, as her first show has sold out completely...