Let's set the scene: You're GMing a relaxing regular weekly game, everything is swell, people are having fun. The characters finally corner the Lt. BadGuyMiniBigBad in some gods-forsaken corner of the dungeon. The epic battle ensues with melee strikes, kung-fu shenanigans and flying spells. All the clues point to one solution - the heroes MUST toss the scepter into the fires in the deep chasm or BIGBAD will be too powerful to defeat. It is also known in some circles that "he who holds the scepter serves the dark one and channels his powers". Of course. The PCs are convinced this is the source of power for Lt. SadFaceSoggyPants so they can eliminate one Lt. AND have a chance of defeating THE DARK ONE. One of the PCs inches closer to the dais...

... and then picks up the scepter and pledges allegiance to the dark one!

What do you do?

Updated details for the discerning;

1. Lt. MiniBadGuy IS using the scepter - that's where he gets his power. The nebulous connection between the scepter and the BIGBAD GUY is yet to be defined. You made it up as they entered the room. You thought it would be cool to have this Lt. to have some physical connection to his boss.
2. You didn't necessarily set the scenario up this way. Maybe you just improved various reactions and this where you all ended up?
3. The player doesn't normally do things like this, but they did mention that they have an evil heart in their background.

Quote from: trechriron;938576What do you do?

Smile gleefully, and thank my lucky stars this is a player in my campaign.

Quote from: Black Vulmea;938577Smile gleefully, and thank my lucky stars this is a player in my campaign.

This.

A whole new campaign has just started with a PC as protagonist and a player that I can use to help co-GM.

Wouldn't that just end in a TPK.

Not enough data.

Is this a table where the PCs regularly pull stunts like this? Is betrayal the norm?

Or is the player trying a double agent ploy to get close to the enemy and use their own power against them?

Or is the player just being a dick? Out of the blue? Or is this part of chronic dickery?

Or is the player playing it as a power greedy type who thinks they can make that power their own?

and so on.

Depending on the details the PC may have just become a NPC, or they may become a co-DM/DM helper. Or player and DM are about to swap places as the campaign takes off in a new direction. Or they get very dead before they can consolidate. Or... and so on.

Oh so many many ways it could go.

I'd turn to the players and ask them: "What do you do?" while secretly hoping that they all follow suit...

Quote from: Necrozius;938589I'd turn to the players and ask them: "What do you do?"

This, and then run things as they occur.

We had a faintly similar situation years ago in which the group confronted an aspect of the horrible deity of death and disease. As they were perparing to brawl with him to save a companion in its grasp, the party Necromancer shouted, "Take me!" to the dark one. So I did just that. Sucked his soul right into the netherworld (and spat it out much later as an avatar and party antagonist). I mean, he DID say "Take me..."

Quote from: cranebump;938593This, and then run things as they occur.

We had a faintly similar situation years ago in which the group confronted an aspect of the horrible deity of death and disease. As they were perparing to brawl with him to save a companion in its grasp, the party Necromancer shouted, "Take me!" to the dark one. So I did just that. Sucked his soul right into the netherworld (and spat it out much later as an avatar and party antagonist). I mean, he DID say "Take me..."

What was his reaction and everyone else's reaction?

SICK GUITAR RIFF would be my reaction.

Then I'd have to decide whether to let the player continue playing this character as an antagonist (which is what I'd be inclined to do), or turn the character into a NPC and have him roll a new one (if the player wasn't cool with epic head-on PvP in grand old Arnesonian tradition).

I would ask the other players "What do you do?"

Because fuck "how the story is supposed to go."

And double fuck "throw the evil dingbat into the Anus of Doom to destroy Lord Killfuck Soulshitter."

And then I hope the players beat seventeen kinds of everliving shit out of me for pulling such lame-ass buttnuggetry on them in the first place.

Quote from: Omega;938587Not enough data....

Oh so many many ways it could go.

Stop fire-walling your creative heart! Just tell me what you would do!!??!! Fill in the details as you desire.

Quote from: Gronan of Simmerya;938599... lame-ass buttnuggetry on them in the first place.

It's just an exercise. No need to place any emotional context on the setup...

Quote from: Black Vulmea;938577Smile gleefully, and thank my lucky stars this is a player in my campaign.

Yeah, this:D!

Quote from: Necrozius;938589I'd turn to the players and ask them: "What do you do?" while secretly hoping that they all follow suit...

And then, this;).

(Come on, Gronan, that's just an example situation!)

Yep, seems like the ball is in the Players' court.
I hand them the ingredients but they make the cake... or pie... or custard tart... or whatever.

I'd introduce a corruption track for the PC.  He now has access to several high-level evil spells, even if his class isn't normally a spellcaster.  Each time he casts one he must make a willpower check or something similar.  A failed roll means he takes points of corruption.  The more corruption he takes, the more penalties he takes.  The penalties may be applied to any combination of the following: non-evil actions, non-evil spells, morally good actions, morally good spells, actions that don't help the Dark One, and actions that actively oppose the Dark One.  If the PC gains too many corruption points he becomes an NPC.  Corruption points can be removed by intentionally abstaining from using the Dark One's spells for a length of time, but that might leave them vulnerable when their enemies attack.  If the player loses the staff he immediately loses access to any of the spells it granted him.