Descent into villainy

Sometimes, a PC starts down a dark path. Maybe they keep using a major arcanum and racking up consequences. Maybe they are corrupted by the Things Below. Maybe they die but refuse to pass on, and linger as a Ghost, Revenant, or Thrall. Maybe they just make one grim choice after another.

It's usually okay for a PC to be wayward or wicked, but not both. If a PC schemes, lies, and causes trouble for the other PCs but their heart is in the right place, that's interesting. And if a PC relies on foul magic or despicable acts, but does so in alignment with the other PCs' goals, then that can be compelling. But a PC who's both evil and opposed to the other PCs is no longer a protagonist. They're a villain.

When a PC crosses the line into villainy, or just cozies up to it, check in. Does this PC's player think they can be redeemed? Do the other players want to try? If not, does everyone want to play out one last conflict or scheme? If the answer to any of these is "No," then the PC should become a threat. If the answer to all three questions is "Yes," then focus on that confrontation in an upcoming session. If the party fails to redeem their one-time ally, then that PC becomes a threat.