View Full Version : Friday the 13th rule

LeoLionxxx

2013-12-10, 10:16 PM

As some people may know, Friday the 13th is fast approaching, a day that is supposedly meant to herald bad luck and ill omens; and what may be fantasy in real life, can become reality in the game world. So, for all you evil GMs out there *cough* all of us *cough*, I present the Friday the 13th Rule for GMs to present to their players and run with on these days of ill fortune:

It's quite a simple rule: upon a roll of the D20, the roles of the 1 and the 13 shall be switched - 1 will be treated as just a normal number 1, while 13 shall be treated as a critical miss/fumble! A perfect way to add just a little bit of bad luck to the adventure :smallbiggrin:

You may decide whether or not to apply this rule to the NPCs of the game as well - after all, if something's making the players have bad luck, it might be affecting the other guys too. Another variation and/or addition can be to give undead creatures (and players I guess) the opposite effect, to treat 20 as just a 20, and 13 as a critical.

Hopefully this rule variant will make your Friday afternoon gaming session just that little bit much more interesting, as your PCs see their "not bad" roll turn into a disaster of some sort. As such, happy rolling folks!

erikun

2013-12-11, 03:10 PM

But I have better luck with 13 than normal.

12 seems to be my bad luck number.

Yay for baker's dozens!

I have triskaidekaphilia.

JeenLeen

2013-12-13, 11:00 AM

It'd be interesting to see someone do the math to figure out how much this rule would change things.

Usually when you roll a 1, the result would be low enough to be a failure anyway. The 1 only makes it a critical failure/fumble/etc. depending on the situation or ruleset in play.

However, a 13 is a medium to decent roll. With a decent save, BAB, or whatever, rolling a 13 probably means a success. As you'd be trading a 'would have failed anyway' with a 'probably could have succeeded', this would decrease the odds of success more than it initially apparent.

Well, I don't have the skill or care to try to figure it all out, but this seemed interesting enough to me to post. :smallbiggrin:

Mono Vertigo

2013-12-13, 01:27 PM

Now that's a rule that's gonna confuse my WoD players. Yes, I know it uses d10, not d20, that's the joke.

Another_Poet

2013-12-13, 02:42 PM

I motion that if the character is wielding a chainsaw, a 13 instead becomes an automatic success/crit.

The Parasomniac

2013-12-13, 03:13 PM

Just to be evil (and hey, it's a one day only thing), I would make any enemy that rolls a natural 1, 3, or 13 succeed in a critical hit or be critically successful in a skill check.

If a player rolls a natural 1, 3, or 13, they will critically fail their attack or skill check.

If you wanted to be even more mean, you could apply the same rules for any roll totalling 13 as well...or any multiple of 13 for higher level campaigns.

Not mean enough? Treat all natural 20's as natural 1's for player characters only.

CarpeGuitarrem

2013-12-13, 03:52 PM

It'd be interesting to see someone do the math to figure out how much this rule would change things.

Usually when you roll a 1, the result would be low enough to be a failure anyway. The 1 only makes it a critical failure/fumble/etc. depending on the situation or ruleset in play.

However, a 13 is a medium to decent roll. With a decent save, BAB, or whatever, rolling a 13 probably means a success. As you'd be trading a 'would have failed anyway' with a 'probably could have succeeded', this would decrease the odds of success more than it initially apparent.

Well, I don't have the skill or care to try to figure it all out, but this seemed interesting enough to me to post. :smallbiggrin:

It actually only (potentially) changes the odds of success by 5%, because it removes one possible success result.

Another_Poet

2013-12-13, 10:21 PM

It actually only (potentially) changes the odds of success by 5%, because it removes one possible success result.

That assumes that all die roll results have an equal chance of succeeding on an action. That is not an accurate assumption.

Craft (Cheese)

2013-12-13, 11:56 PM

That assumes that all die roll results have an equal chance of succeeding on an action. That is not an accurate assumption.

It never modifies the chance of success by any amount except 5% or 0% (unless you're factoring in rerolls in which case it gets slightly more complex). If you can only succeed on a 14 or higher, then the rule does absolutely nothing. If you can succeed in a smaller result, it decreases your chances of success by 5%.

1337 b4k4

2013-12-14, 10:33 AM

It never modifies the chance of success by any amount except 5% or 0% (unless you're factoring in rerolls in which case it gets slightly more complex). If you can only succeed on a 14 or higher, then the rule does absolutely nothing. If you can succeed in a smaller result, it decreases your chances of success by 5%.

To elaborate further on this for basic single d20 rolls, a d20 has an equal chance of rolling any number on the die.

The in easy case, on any roll where you would not normally succeed on a 1 (say a DC 10 roll) the rule change simply changes the possible success values from 10-20 to merely 10-12, 14-20, or a 5% decrease in your success rate.

On any roll where you would not normally succeed on a 13, the rule change has no effect. Failure is failure regardless of whether it's by missing the DC or natural roll fail.

The final scenario is where if there was no natural fail roll, you would succeed on a 1. Again in this case there is no change in outcome probability. The natural 1 rule implements a guaranteed 5% chance of failure. The rule change simply changes which number generates the guaranteed failure, but it's still a single number and therefore a 5% chance.

Jlerpy

2013-12-15, 05:33 AM

Now that's a rule that's gonna confuse my WoD players. Yes, I know it uses d10, not d20, that's the joke.

It'd rather mess with my GURPS players' heads too...

CarpeGuitarrem

2013-12-16, 12:35 AM

It never modifies the chance of success by any amount except 5% or 0% (unless you're factoring in rerolls in which case it gets slightly more complex). If you can only succeed on a 14 or higher, then the rule does absolutely nothing. If you can succeed in a smaller result, it decreases your chances of success by 5%.

Bingo. From a pragmatic perspective, you can just look at it and say "at most, it reduces your chance of success by 5%" and note that 5% really isn't all that much. So it doesn't screw up the math badly.

(That being said, it will feel painful when you roll a natural 13.)

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