Huh. Nobody's asked that before.
I've never posted it in a single, concise post anywhere: you'd be able to piece it together from looking at at blog posts going back to around August 2017 so and looking forward from there, but to save you the trouble, I'll summarize here:
The objective of the Matter build is to add a new ability type, Matter, to Demon. Matter will cover ground that really didn't fit under the domains of any of the existing elements: things such as acids, petrification, entangling, disintegration, and the like. The goal is for Matter to feel like it was always part of the game; thus, it will be as fully supported as all the existing elements such as Fire, Ice, Body, etc. Ideally, a new player would never be able to guess Matter was not always part of the game.
To this end, 20 new characters and nearly 80 new abilities are being added: the new characters will be spread amongst the entire game, and be the main way new Matter abilities are introduced into the game. However, several existing characters are also having their ability sets adjusted a bit to even out the distribution and help Matter be more smoothly integrated into the game as a whole, rather than being clumped only on the newer characters.
Finally, we should not forget all the other little details necessary to make Matter feel like an original part of the game: character creation options, demon modifiers, updated resistance tables for all characters that make some weak/resistant/immune to Matter.
As a minor aside, the Matter build also brings the total recruitable demon to count 153, which makes me happy for the irrational reason that the first version of Pokemon had 151 Pokemon. It's a nice little milestone to have passed.