I'm working on a simplified RPG system that uses only D6s, and I want a mechanic for fumbles/critical fails.
Depending on how good the player character is, they have 1-5 dice to roll and they have to beat a difficulty set by the DM. I thought it would be fun to have players fail if they roll all 1s, but realized it makes it way too hard to fail if you have 5 dice, and a bit too easy if you have 1. Is there a more linear way of defining critical fails?
This is what I get if fumbles are on all dice showing 1s:
| Number of Dice | Probability of Fumble |
|---|---|
| 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):
| Number of Dice | Probability of Fumble |
|---|---|
| 1 | 18% |
| 2 | 15% |
| 3 | 12% |
| 4 | 9% |
| 5 | 6% |
Best Answer
Fumble if (\$2 \times\$number of dice) \$\gt\$ (sum of dice except for any ones)
Equivalently: (number of ones)\$+\$ (\$2 \times\$number of dice) \$\gt\$ (sum of dice)
This is the most "linear" rule I've found without sacrificing too much practicality. When searching for a strategy, I wanted to avoid looking at any properties involving specially-marked dice or "ordered" dice. I limited myself to looking at linear sums of either the total number of dice, the counts of different values, or the sum of all the dice.
Assuming that the players are going to be calculating the sum of the dice anyways, they can check for a fumble by subtracting the number of 1s that were rolled, then subtracting twice the number of dice. If the result is negative then that is a fumble.
Edit: If you haven't finalized a decision on how exactly the dice rolls will determine success / failure, then I would recommend using "the sum of all dice excluding any 1s" as the value to be compared against the DM's target number. This way there is good integration of the above fumble mechanic into the rest of the dice-rolling mechanics.
Fumble when (count of ones) \$\gt 2 \times\$ (count of fours, fives, and sixes)
The above rule is another practical rule I've found. A player can look at the dice results, count up how many 4/5/6s were rolled, multiply by 2, then subtract how many 1s were rolled. If they end up with a negative number, that's a fumble.