The Setup: the PCs already had one encounter with the villainess, during which they managed to steal an inscribed ceremonial knife; it also functions as a normal dagger. While planning for the climactic battle, the PCs learn that this knife is the only thing that can kill her.
The Twist: the villainess wants to be killed with this knife in order to activate the evil plan. She’s trying to be possessed by Hecate the goddess of undeath, who will only enter a body that’s been “purified” with the correct ceremonial weapon.
It’s is a twist on the “villain gives her plan away in the monologue” trope; the information she’s giving out is a deception! (I mean, what else would you really expect from the villain?)
|“If you hadn’t come, I couldn’t go. Think about that…”|
This con has three parts.
Part #1: The PCs come into possession of this knife.
Probably by snatching it from her person or stealing it from a secure area. It’s important that they are stealing it from her, not just picking it up somewhere – her misinformation in part 2 is an attempt to get it back from them, after all, not just hints she’s dropping in hopes they’ve found it lying around.
Part #2: the Misinformation
The PCs hear the villain say this knife is the only thing that can kill her, or the only thing that can disrupt their plans (technically true – the PCs having it instead of her having it is the only thing that could disrupt her plans). The PCs can…
- Overhear her commanding her minions to hunt you down you and get the knife back. “It’s the only thing that can harm me!”
- Learn about it in an information-gathering scene. Perhaps an ally has a book that talks about the “only” thing that can kill a necromancer who has this certain enchantment (but of course fails to mention what would happen next)
- Be captured and told by the villain herself. Perhaps she even offers them a bargain where she’ll let them go free if they give her knife back.
Part #3: The Twist
“The knife sinks into the witch’s heart. She staggers, crumples, then slowly stands up. ‘Fools! Did you think Hecate would deign to enter a living body? By your own hand you have opened the way to my ascension!”
…you have to make this one dramatic, or the players will likely see it as a cheap way to of making your villain invincible.
When I pulled this knife trick on my players during the final session of our then current campaign, I had a moment where I thought they were going to kill me. Instead, their shocked silence turned to excitement as they decided what to do next.
I think you need two things to do the Trick Knife well. First, I described it in a very cinematic way – if you describe it so they can see something awesome in their head, they’ll usually jump on board with it. Second, they weren’t completely screwed. Sure, I’d “tricked” them into inadvertently powering up the bad guy and making their job a lot harder, but this was an instance of good railroading: the kind where you give them something really challenging and exciting (remember to work that description!), so they get pumped about overcoming it.