Planescape is a silly place.
Dungeons & Dragons is a wholeheartedly silly game, and it's important to remember that what makes it silly is an expansive growth out of a particular root. It is a tree of many branches but thanks to the way that it encourages people to build their own things on top of it, it has become a sprawling kind of folk narrative and generally accepted consensus material that then a company comes along and tries to augment and supplement. Still, as much as a corporate mind is at the head of what gets published, what gets handed to that corporation is going to derive from the mind of a dork who likes D&D. To that end, D&D's lore is a constant push-pull between the kinds of nerds who like organising lists and the kind of nerds who like to invent new types of dragons they want to have sex with and they're all trying to integrate one another's material because that's how nerds demonstrate mastery over a topic.
The result is that D&D lore is composed of parts that neatly and smoothly fit together and parts that should be airbrushed on the side of a van, and all subjects exist in a space between those two points, on a spectrum. And nowhere is this more evident than in the way that 2e's setting Planescape introduced elements that 3rd edition tried to hide.
Planescape, as a setting, exists very close to the 'airbrushed on a Van' side of things, and it's extremely obvious when you look at its roots in 2nd Edition. In this space, much of what makes Planescape Planescape was codified. For those of you unfamiliar, Planescape is a setting made up of the idea of 'planes' as distinct, discrete universes with their own rules separated not by time and space, but just by barriers or magical boundaries. You know how Narnia is supposed to work, with the wardrobe? It's like that, but there are a lot more wardrobes and they all go to different places. Think a sort of multi-level Isekai scheme.
Anyway, it's a setting with like, multiple whole universe-sized worlds, that may or may not have planets inside them, some of which follow a very narrow set of identifying rules, like the elemental plane of Fire, which is full of Fire, or are just like 'here, but a bit weird,' like Bitopia, which is a whole plane that is mirrored vertically at a certain height. If you look up in Bitopia, you see another whole country up there - that's why it's called that. Also everyone there is bisexual.
Planescape sought to build out more of that structured universe and then in each structured space, fill it with interesting notions. But the structure is a little odd, in that it's hard to make an infinite number of chairs organise neatly, someone is always putting out one more where they shouldn't. That means there are tidy diagrams of the Planar cosmology, and then you look inside any of the bubbles in that diagram and find it's full of gibberish.
It was in 2e that, as far as I know, we were introduced world-wise, to the characters of the Modrons.
There's a whole writing form that involves referring to Modrons in deliberately obtuse ways, with Modrons being the individual, plural, categorical, and utility terms for this people, but what you need to know about them is that Modrons are weird lil guys that are made out of a basic geometric shape - pyramid, cube, dodecahedron, all the way up to sphere (or down to sphere, depending on who you ask). They are truly perfect Lil Guys, a byproduct of a plane of true law and order which doesn't in any way cohere to what humans (the people playing the game) necessarily assume about law.
They make a lot of sense in a storybook kind of way where you don't need to have big answers for what they are or how they work or even how their philosophical bias towards pure lawfulness works. In the world of 2ed, where sometimes things that sound like they should be well explained, clear rules are kinda yada-yada-yada'd in a space that you might imagine is flavour text, the Modrons left a bunch of questions unanswered and seemingly, that was good. It was good that they were heavily ambiguous because what was the life cycle of 'an orb?' Any answer made them less mysterious and pushed them away from the oddness that they represented.
Anyway, 3e was an attempt by a serious company to do serious things and that's why when they went back to talk about the Creatures That Lived In The Lawful Planes, they came up with the Inevitables.
Inevitables are the demons of small minds, writ large. Literally, the point of an Inevitable is to be a Lawful Neutral version of a Demon, an entity that exists purely based on rules, coalesced out of a world made of rules, and with nothing holding them back from expressing that. Each of the Inevitables is meant to respond to a rule in the universe and then enforce it. They are self-appointed near-immortal construct cops, and they're meant to oppose things and people that break the rules that they, specifically, are meant to care about.
These rules are completely out of whack, though, because one of them is meant to enforce say, justice, another the inevitability of death and another, the way the desert is a fixed ecosystem that nobody should try and change or interact with. And in that case, there are a bunch of plants that the Inevitables are going to have issues with, that don't seem to be capable of forming complex political allegiances.
There's a really interesting distinction between Inevitables and Modrons, to me. Modrons are weird and interesting but also, there's nothing they can do that answers a question. Inevitables are a fun challenge that's supposed to be present to oppose players or potentially be recruited into an adventure, but not for too long. But Inevitables, the 3e attempt to populate Lawful Planes with A Kind of Guy, sort of fell apart and are now more of a trivia question while Modrons have endured into 4th and 5th edition.
I don't think there's some greater, better reason for it or anything. I don't think that Inevitables failed because they were Bad Design or something. But I do think that for me, the way that Modrons represented Weirdness was much more interesting than the ways the Inevitables sucked weirdness away with their simple, clear consideration of certain things as being part of natural reality.
After all: Inevitables would hunt down people who extended their lifespans because 'everyone must die.' But Inevitables were immortal. That's a pretty interesting thing to juxtapose and maybe a character could struggle with that.
Or maybe they could make a big speaking trumpet and demand that everyone else refer to them as a Spokesmodron which is, in my opinion, much funnier.
