1of3 said:
I'm still not sure what either kind is good for. What kind of adventures would I use them in? What's their role?
Both fall into the camp of "difficult-to-deal-with, not-necessarily-hostile NPC's"
Modrons are alien, weird, and authoritarian. They are the kind of thing the PC's meet when you want to enforce how utterly and fundamentally
different the other planes can be (think: "
Blue and Orange Morality"). Inscrutable and largely indecipherable, modrons seem to adhere to a logic, but a logic that is difficult for those not part of their network to understand. They should inspire a sort of uneasy laughter in the players, a desire to tread carefully. These things are weird, but they're
powerful, and they're numerous, so if you set off one of their triggers it could go badly for everyone involved, and you're never entirely sure what might trigger them. They can be useful allies if your purposes are aligned, but never entirely safe beings, never something you can really predict the actions of.
Slaadi fit a similar role, but rather than a sort of institutionalized madness, they are more of an affable destructive force. Dangerous, prone to violence, and always looking to cause trouble and screw the system, they disrupt the world wherever they tread. This disruption isn't always negative, and can even be fairly positive, but it's always quite traumatic. You can't trust a Slaadi because you know their allegiance is always and ever only to themselves, and their own desires are to fulfill whatever whims come to their mind. They're powerful enough that few others can reign them in, so they largely treat the universe like some sort of cosmic plaything, like a kid with play-doh, destroying and re-forging it. PC's who encounter Slaadi should have a similar reaction to PC's who encounter modrons, but with modrons it's a lot of talking and negotiation and understanding (if you get them, you can use them well). With slaadi, it's a lot of arm's-length persuasion and a constant readiness to change tracks with them. You'll never get a slaadi. The best you can hope for is to go along for the ride.
In both cases, they fill a role similar to a lot of the old stories of the Fey: weird, otherworldly, powerful, incomprehensible, but not exactly
hostile, either. Just different. Dangerous, but different.
If you want to "three pillars" it, they're primarily an Interaction challenge. Fail and they will become enemies in combat and will hurt your goal, succeed and they will become allies and will help your goal.