In D&D, modrons of more than one rank above or below the modron are too complex or simple to be understood. Rouge modrons might be able to comprehend even more, but what about players? This variant rule allows a character's intelligence determine their comprehension of the modron language. Essentially, the modron language has set levels of complexity that characters can only learn if their intelligence is good enough.

Here is the rule: Each type of modron is associated with an intelligence score or intelligence range. If you fall within a modron's range or meet the score associated with the modron, you can communicate with modrons of that rank and one rank lower or higher. Some modrons like pentadrones and quadrones share a range, meaning one that has an intelligence of 10 could comprehend pentadrones, quadrones, tridrones, and decatons. To understand a modron two ranks away from the 'rank' you best understand, you need to make a DC 10 Intelligence (Insight) check, the DC of which increases by 5 for every additional step you take up or down.

Here are the ranges and intelligence ranges associated with it:

Monodrone: 3 - 4

Duodrone: 5 - 6

Tridrone: 7 - 9

Quadrone: 10

Pentadrone: 10

On the DM's Guild, Michael Crary created statistics for hierarch modrons. His supplement is a pay-what-you-want document, meaning that you can get it for free or support his great work. Either way, using his statistics, you can expand the intelligence charts. If you aren't using hierarchs in your game, associate an 11+ intelligence with the pentadrone:

Decaton: 11

Nonaton: 12 - 14

Octon: 15

Septon: 16

Hexton: 17

Quinton: 18

Quarton: 19

Tertian: 20

Secudus: 21 - 27

Primus: 28 - 30

Note that the Micheal Crary document seems to have an error where Quintons, a lower ranking modron, has an intelligence higher than a quarton. I've chosen to disregard this for sake of realism. I do not blame Crary for this error, because I know that Wizards of the Coast themselves have made similar mistakes; In Mordenkainen's Fiendish Folio lists Norkers, a small creature, as having d8 hit dice.

Now for an example: Bob has an intelligence of 10 and can either understand modron or cast Comprehend Languages. He can comprehend quadrones, pentadrones, tridrones, and decatons in the modron language. He must make a DC 10 Intelligence (Insight) check to comprehend duodrones or nonatrons, a DC 15 check to understand monodrones or octons, a DC 20 check to comprehend a septon, a DC 25 check to understand a hexaton, a DC 30 check to understand a quinton, a DC 35 check to understand a quarton, and so on, all the way up to an impossible DC 50 check to understand the speech of Primus. Now, some upper level modrons would likely speak Common, and modrons above the rank of monodrone are outfitted with the ability to speak Common when they go on the Great Modron March, so you'd rarely speak with a modron hierarch in modron unless you wanted to speak without the understanding of a non-modron listener or spy on a conversation between modrons.

Nice.