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I'm working on a simplified rpg-system that only use D6 and I want a mechanic for fumble/critical fail.

Depending on how good the player is they have 1-5 die to roll and they have to beat a difficulty set by the player. I thought it would be fun to have players fail if they roll all 1:s but realized it makes it way to hard to fail if you have 5 die, and a bit to simple if you have 1. Is there some more linear way of defining critical fails?

This is what I get:

Nubmer of Die   Probability of all 1:s
1               16.67%
2               2.78%
3               0.46%
4               0.08%
5               0.01%

What I would like(approximately, exact numbers are not that important):

Nubmer of Die   Probability

1               18%
2               15%
3               12%
4               9%
5               6%
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Unless you want those exact percentages, one simple possibility is that the player must roll a one on a d6. Then, if it's difficulty level 1, they're done; if it's difficulty level 2, they must roll the d6 again and get 1-5; if it's difficulty level 3, they must get 1-4; and so on. The actual percentages would then be 16.7%, 13.9%, 11.1%, 8.3%, and 5.6%, but it's close and in the right proportions.

ETA: Sorry, I just re-read the problem and realized this is for critical fails. So it's not really difficulty levels, but the same game action should work either way.

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With one D6, you can only have multiples of $\frac16\approx 16.7\,\%$ as probabilities.

With two dice, $15\,\%$ of the $36$ outcomes is $\approx 4$ outcomes, so fail on "both are $\le 2$"

With three dice, you want to fail on about $26$ outcomes. It is easy to achieve $27$ outcomes: Fail on "each die is $\le 3$". In order to really have $26$ outcomes, try "... but succeed on three threes"

With four dice, you want about $117$ failing outcomes - I do not see a really simple description of such an event; similar for five dice and $\approx 467$ outcomes.

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