[3.5e] I've plotted myself into a corner

[Fishy]

"Dark Lord Tyrannicus has to be stopped, or he'll raise import tariffs and artificially inflate the cost of doing business, leading to short term gains but a gradual economic cooldown! He's Mad, I Tell You!"

Ahem. What I mean to say is, if you want the plot to end with an epic battle rather than several months worth of court cases and senatorial probes, the villain has to be up to something that justifies violencing. I agree with Starshade: you need a Kefka moment. Something that pushes your players over the edge from 'Wow, he's kind of a jerk', to 'Merciful Gods, he's inhuman, and he's going to kill us all'.
Idea:

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I like the doppelganger idea- has a sort of Baron Wulfenbach vibe to it, which I always love- but why not go one step further and make them Pod People?

All those political and military leaders that are convinced he's such a nice guy? Some of them are still human, but most of them are talking about themselves. The BBEG has been systematically increasing his influence, getting private audiences with powerful people, and eating their brains. Those 'assassinations' he's been implicated in? They're the ones who realized what was happening and killed themselves before he took over.

He's not invading. He's expanding. He's infecting.

Mechanically, there are about a dozen D&D monsters and spells for taking over minds and bodies. Might be something obscure, might be something as simple as a vampire or some other Spawn Creating undead. Which would give him a good reason to put the good aligned churches under his thumb, Turn Undead ruins your whole day.

So, what happens next? Well, the MBEG has to be still human, or she won't defect. She probably doesn't know exactly what the BBEG is or what he's up to, but she's worked out that he has some kind of method for turning vehement enemies into devoted allies after one private face-to-face visit. Maybe he tried to turn her, and she escaped with her stealth abilities, and that prompted him to discredit her before she could do too much damage.

But, that didn't work, now the PCs are after him, and he has to kick it into high gear if he wants to live through this. He picks a convenient town somewhere between his stronghold and the PCs current location, makes a personal visit, and has a busy night. The PCs arrive, settle in to the tavern for the night- and wake up with a kitchen knife in their shoulder and every single man, woman and child for a mile around trying to kill them. Cue the escape sequence, exit in mad rush to find and kill the original and hopefully free his mindslaves.

A bit cliche, but fun? Maybe?