So, I've been running GPC for well over a year now, and my group is nearly in the Twilight Period. However, I'm worried about the upcoming event where Lancelot goes on a rampage after having been caught with the queen. His sword skill has been raised to heavens in the scene, and he can auto-crit two people in a single turn instead of one. Two people aren't a problem in this case, a single person is.

One of the PKs has been putting nearly every single point of his impressive amount of Glory in to his Sword skill and as long as he doesn't manage to get himself killed beforehand, I've no doubts it will be at least 34 around the time Lancelot throws a fit, and he can further increase it with his passion Loyalty (Arthur), which is around 20 I believe. I don't remember if it's from an errata or just one of our house rules, but in the case of both combatants getting a critical we've ruled that they both deal 1d3 damage to each other unless one of the was fighting defensively in which case he doesn't deal any damage.

Unless the guy has some really bad luck in the next few aging rolls, he's going to have more hit points than Lancelot when the fight happens, and doing the math I figured that he just might beat old Lancy. This of course would be a huge wrench in the machinery for the story.

What should I do? I can't very well tell him that he just can't fight Lancelot. I also feel like trying to railroad him away from Camelot for the night would cheap.

Give Lancelot more hit points.

Short answer: Let him kill Lancelot. He has 'earned' it by actually managing to make a character that can do it.

Longer answer: Lancelot is not unkillable, especially not at that late stage. You can easily adapt the storyline by having the rest of the De Ganis knights revolt over the death of Lancelot, and have Bors step up from Lancelot's shadow and lead the rescue of Guinevere ("It is what Lancelot would have wanted."). Bors kills Gaheris and Gareth, causing Gawaine to swear a bloodfeud. Bors and Gawaine square off in a duel, but Bors is younger than Gawaine and managing to match Gawaine in skill, wounding Gawaine badly. In short, you can just run the rest as it was, replacing Lancelot with Bors, and giving the player the satisfaction of killing that 'shinier-than-thou' Lancelot. :P

I would seriously NOT railroad the PK off the situation, if he has a 'right' to be there. On the other hand, is he pals with the Smug Snake Mordred and the Proud Agravaine? They come of quite bitter and anti-Arthur in the books, and if the PK has Loyalty Arthur 20, he might think less well of Mordred who is trying to subvert Arthur's authority with the Blood Tournaments and the like. Thus, Mordred might not trust the PK enough to invite him along: surely Mordred wishes to have knights he knows will back his play no matter what it is... But if the PK is a character that Mordred would ask along, then by all means let him come and kill Lancelot if the dice gods allow him to do it!

Mind you, I probably would give the critically inspired Lancelot the benefit of 'fight until you die', allowing him to fight through the unconscious threshold until he actually hits 0 or negative hit points. After all, he needs to kill all the witnesses or the woman he loves & is critically impassioned for will die horribly. Also, if the PK does manage to nick and whittle Lancelot to 0 hp or below, first aid will probably save his life well enough, unless the PK coup de graces him (if he does not, I'd have Mordred do that unless the PK stops him). If Lancelot survives, this might end up with Lancelot being led to the pyre as well (or executioner's block) at the same time Guinevere is, and being rescued by Bors. Lancelot gets a sword from Bors / one of his dead captors, cuts his way to Guinevere, killing Gaheris and Gareth, and you are back to the canon, but have let the PK get the benefit of his skill. '

EDIT (after Greg's comment): -AND- you can hang all the bad stuff on him afterwards if he decides not to coup de grace Lancelot: "Oh, if only you had killed Lancelot earlier, then Gawaine, Gaheris and Gareth would still be alive, de Ganis rebellion crushed or not starting at all with Lancelot safely dead, and Mordred unable to seize the power." Why yes, I am an Evil GM; why do you ask? :P

I agree with Morien

...and I agree with Morien and Greg. If Lancy kicks the bucket, then though titty. Your player earned it, and it's cool to see people's GPCs go in all sorts of different directions. On the other hand, if you're not confortable with it, or if you prefer to keep the "canon" more or less predictable with the PKs just filling in the blanks, then you may have to tinker with Lancy's stats or do some railroading.

Thank you for the advice!