House rule for a game more lethal ad full of (usually nasty) surprises

[Giovanni81]

I have played a bunch of combats and the game runs very smoothly.
I wonder if substituting 2d10 with 1d20 some fundamental mechanics will broke or not.
Of course, with 1d20, I will maintain:
19-20 => reroll and sum
1-2     => fumble

[Giovanni81]

Ora maybe fumble only with the result 1.

[Fidoric]

Hello Giovanni.
I guess you won’t be breaking anything if you use 1d20 instead of 2d10. Dice results will fall under the same range although not under the same distribution. Results with 2d10 tend to me more dense around  the average whereas 1d20 as a linear distribution. Basically you will have extreme results more often with 1d20.
Regarding your propositions, keep in mind that every result on 1d20 has the same probability of occurrence which is 5%. Fumbles under FX rules happen on a double-1 which occurs only with a 1% probability. Fumbling on a 1 or 2 as you propose makes characters fumble 10% of their rolls. A bit high I think but it’s up to you.
Same thing with the exploding dice. Dice explode on natural 19 or 20 in FX which is 3% or possible rolls. The same range with 1d20 occurs 10% of times.
Using 1d20 with special results on 1 or 20 would be closer to the rules as written. What you propose will end up with 20% of rolls qualifying either for a fumble or an exploding roll. If it works for you, that’s fine.

[Giovanni81]

Yes, in summary:

1. PC will face death more often.

2. Low skillled enemies will remain relevant for longer time.

3. There is also room for positive surprises: they will also happen more often.

1,2 and 3 are welcome in my wargame sessions since I search to maximize deaths and situation-changing rolls ..... but for a weekly RPG game: they can be interesting only for a campaign very OSR-oriented.

Fumble: I will remain with "only 1 trigger combat fumbles no matter the weapon".
Explosions: I will maintain 19-20 since open ended rolls are fun.

[Rasyr]

As Fidoric says, the chance of any given number on 1d20 is 5%

For 2d10, the chance of a given number is as follows:

2 = 1%
3 = 2%
4 = 3%
5 = 4%
6 = 5%
7 = 6%
8 = 8%
9 = 9%
10 = 9%
11 = 10%
12 = 9%
13 = 8%
14 = 7%
15 = 6%
16 = 5%
17 = 4%
18 = 3%
19 = 2%
20 = 1%

Since open-ended high are on 19-20, that means there is a total 3% chance of open-ending a roll.

Yes, I would change fumbles to just a 1 and open-ended to just 20, since there would be a 5% of either

This will mean that the results will be a bit more random, since every result has the same chance, but it should still work.

[Giovanni81]

Thanks: I'll go your way.

[Rasyr]

Thanks: I'll go your way.

Do what feels right for you, it will likely work out better for you.

I was only trying to offer advice (which you are NOT required to take hehehe), and present you with options and with some of my reasoning for using 2d10 to begin with.

When I first started working on Fantasy Express, I was actually going with a d20 for the mechanic, but then later changed to 2d10, mostly because I like the bell curve (or perhaps it should be called a pyramid curve or a bell pyramid hehehe), and how that had a greater chance of providing averaged rolls (which helped in other aspects of  the game's design by giving me a known average rather than an equal chance for each result)