Apr 2, 2019

Flipping the Core Mechanic of Mythras

I'm prepping for another Dawnlands campaign, and one aspect I'm debating is flipping Mythras' core mechanic around and simplifying it.

Mythras is a typical BRP game in that all skills have a percentile rating which one attempts to roll under on 1d100 to succeed. Critical successes occur when one rolls less than 1/10 of the skill's percentile rating (rounded up). Regardless of skill level, a roll of 01-05 is always a success, and a roll of 96-00 is always a failure. Fumbles occur when the roll is a 99 or 00. When two characters make opposed rolls, the one with the higher result within the same band of success (regular or critical) succeeds.

This isn't hard, but it's more complex than it needs to be. As Delta has pointed out in designing the Target20 system for OD&D, based on a commonplace amongst math educators, addition is the least demanding and most accurately performed mental calculation. Because of some of the fine details, logjams tend to occur in a couple of places in the current Mythras skill algorithm.

The first place is understanding that bonuses and penalties change the percentile rating you're rolling under, not the roll itself. This takes people a bit to wrap their head around.

The second is that you're trying to roll under a skill rating, but as high as possible within that band. This is sometimes called a "blackjack" method - you're trying to get as close as possible to the rating without exceeding it.

The third is determining which band of success you're in for certain low rolls. It's very easy to confuse a high critical success roll with an extremely low standard success roll, especially when you start adding on difficulty grades, augmenting skills with other skills, and all the other stuff.

To resolve many of these issues, I'd like to offer my simplified Mythras skill resolution system, which I am tentatively calling Target101.

1d100 + skill value + difficulty value + augmenting skill with a result greater than or equal to a value of 101 is a success. If two rolls are opposed to one another, then the higher result wins.
Successful rolls (not total sums) ending in 5 are critical successes. 

A roll of 01 or 02 is a fumble.
A roll of 00 counts as 100.

NB: I'm electing to go for the static difficulty values per the "Simplified Diffcult Grades" table on page 38 of Mythras.

The advantages of this system are that it involves a sequence of simple 2-digit additions that sum to a single score. It reduces or maintains the range of possible exceptional rolls (fumbles, automatic successes). And it allows a simple one-digit recognition operation to determine whether a roll is a critical success or not. The chances of success are mostly mathematically identical to stock Mythras, with only the range of automatic failures and successes reduced.

I haven't play-tested this yet, but I'm hoping to in the upcoming Mythras campaign.


  1. im not very familiar with mythras so give me some rope here, but on looking at the difficulty grades you could continue with the the Target20 analogy by making them all positive as well, somehting approaching +0/20/40/60/80/100 or +0/10/20/30/40/50/60 (though that does change the spread from exponential to linear, of course; im unsure how much of a change gameplay-wise that would turn out to be).

    Past that, looks good, it certainly simplifies the headspce problem regarding keeping the numbers straight

  2. This will depend on the math skills of your group, but for me and the ppl I play with, two digit addition is much more taxing than comparing numbers.

  3. I use a modified OpenQuest in which you have difficulties of -25/-50/-75. No nickle and dime tiny percentages. Then the Harnmaster critical system of criticals on anything ending in a 0 or a 5 (which I believe makes it 20% but that's fine, I like it exciting).

    The math is thus very simple. Roll Skill +/- difficulty or lower on d100 with criticals already determined without math.

    1. I am strongly tempted to adopt the Openquest modifier bands as well. I'm a big fan of them. Because of the way difficulty in Mythras works, I'm also tempted to try out +66/+33/+0/-33/-66/-99 for the modifiers as well, but I haven't done the math to see how closely that sticks to the existing mechanics and expectations of difficulty.

    2. You might consider the Harnmaster bit to remove the percentages. Personally I think that was an enlightened bit they came up with and I'm amazed other games never adopted it. I assume because it goes against the low role is good/high role is bad paradigm.

  4. I thought this was a cool and interesting post. Just wanted you to know I liked it and linked others to it today. I will be interested in seeing where you take it.

    1. Thanks! I've written thirty pages of campaign prep already, and am going to be seeing the gang tonight to set up the campaign. I'll post more once it's under way. :)

  5. I'm planning to something similar for my next Dark Hersey game, it should make determining Degree's of Success much simpler.

    1. I first learnt about this method of d100 task resolution when I was running a Dark Heresy 1e campaign, and it was a tremendous success in simplifying things. That success is part of what motivated me to do it here as well.


  6. WEB EASY is a leading Web Development Company in Pakistan.They are well known in this field SEO Services Pakistan. Their services include WordPress theme development, Custom web development, and Digital Marketing services.WEB EASY is Digital Marketing Agency Pakistan specializing in web design, web development, e-commerce.
    SEO Agency Pakistan
    SEO Provider Pakistan
    SEO Packages Pakistan
    Web Development Company