My campaign theme is Law vs. Chaos.  There are two BBEGs: a pair of elves trying to make the Material Plane into a chaotic paradise, like the Feywild, and a group (cult?) of dwarf clerics of the Order domain, trying to make the Material Plane into a place of absolute law, similar to Mechanus.

However, confronting those two forces directly is far in the future.  For now, I’ve been trying to think of monsters/enemies that are particularly linked to either Law or Chaos; thus far, I’ve only been able to think of a few, such as devils and demons.  Since I’m also planning adventures and building dungeons at the same time, I would really appreciate any ideas that would reduce the amount of research I need to do!

As it stands, I have three ideas which I plan to implement: a cult dedicated to Mammon, a coven of hags, and a lawful neutral priestess of Helm, who will probably join the Order of the Law, unless the PCs find a way to convince her otherwise.

Any ideas?

You have a rare opportunity to highlight the underdeveloped rivalry between the slaads and the modrons.  If they're even aware that they have a rivalry.

 An interesting possibility if you play it right is to use the duergar and drow rivalry. The duergar are opposed to law but are also opposed to dwarves, while the drow are neutral but might swing more to the elven chaotic ideals if forced to pick sides.

I’m sure the sides of Chaos could attract some satyrs!

(Although I guess I just made a lawful neutral satyr...so who am I to talk)

This may be a stupid question, but have your read Michael Moorcock?  I mean his fantasy writing put forth "law 'vs' chaos" (the 'vs' in quotes because the conflict isn't that simple an opposition) as a way to upend Tolkein "good vs evil," and was a big influence on the DNA of D&D. It just seems if you're going to do that conflict, you might get more from "the" classic stories about that conflict than poking the message board.

Kobolds vs Dragonborn.

Generally, Kobolds are free-thinking and individualistic revolutuionaries (chaotic), while Dragonborn are dogmatic, team-oriented traditionalists (lawful).

In my current game, the Dragonborn have a horrible secret, one they would kill to protect - Dragonborn and Kobolds are related. Having the tworaces on either side of a big conflict would be great (arguments between siblings are always more dramatic!).

Quote from Greenstone_Walker >>

Kobolds vs Dragonborn.

Generally, Kobolds are free-thinking and individualistic revolutuionaries (chaotic), while Dragonborn are dogmatic, team-oriented traditionalists (lawful).

In my current game, the Dragonborn have a horrible secret, one they would kill to protect - Dragonborn and Kobolds are related. Having the tworaces on either side of a big conflict would be great (arguments between siblings are always more dramatic!).

 I’ve already made my Kobolds lawful; they have a fairly advanced society under my capitol city, which pays them in food, goods, and protection to dig and maintain sewers.  Not having to scrounge for food or hide from threats, they have begun to emulate the culture of the city above them.  I like the idea, though!  Maybe I’ll try to reconfigure your conflict for my world.  It could be interesting to see what happens when they meet the Dragonborn PC!

LAW: Use Formians (the ant guys), Inevitables (don't use the shitty one included in Volos or Mord's, homebrew your own Kolyaruts, Maruts, etc), Modrons (seriously these guys dont get any love)

CHAOS: Whoever said Slaad, I second that. Love them. Maybe make some anarchic elementals or something. I keep forgetting that 5e lacks all the cool templates 3.5 had. Shame.

You could go blood war. Demons vs devils. Or at least have it make an appearance.

Devils vs demons is always a classic, but the BIBLICAL storyline would be the devils AND THE ANGELS having to reunite as one Host again to preserve the universe from Chaos.  The after-effects of that (especially how it affects Asmodeus's fortunes) are your whole next campaign.

Since someone mentioned Fomorians, contrasting them with the Thri-Kreen could be really cool; the co-operative ants against the more solitary mantids.

Quote from TimCurtin >>

Devils vs demons is always a classic, but the BIBLICAL storyline would be the devils AND THE ANGELS having to reunite as one Host again to preserve the universe from Chaos.  The after-effects of that (especially how it affects Asmodeus's fortunes) are your whole next campaign.

Since someone mentioned Fomorians, contrasting them with the Thri-Kreen could be really cool; the co-operative ants against the more solitary mantids.

 If it wasnt for the inevitable political and religious flamewars it would induce, biblical inspired campaigns/stories could be terrific, but theres a lot of walking on thin ice. Although I see the force of Chaos itself being that of Lucifer, given that he's very prominently the antithesis to God. I also like turning up the alien nature of angels up to eleven. Sure, they might look like you but winged, but they should ultimately have a sense of morality very far removed from mortal ones.

In fact, I like this idea.

FORCES OF LAW: Angels, who seek above all to restore the universe to its natural form and to expunge the forces of chaos from it, at any cost, even if it means destroying the world and everyone in it.

FORCES OF CHAOS: demons, who seek to conquer the Crystal City and replace God as the sovereign.

Stuck between a rock and a hard place.

Orcs and Goblinoids.  Orcs are traditionally Chaotic Evil.  Goblins are Neutral Evil, Bugbears are Chaotic Evil, and Hobgoblins are Lawful Evil.  Hobgoblins are the ones in charge, Goblins the more numerous, Bugbears the least.

Simple, well known, common ordinary monsters.  No need for things from the outer planes.

Orcs would make superior troops if discipline could be maintained.  They are big, strong, and tough.  Goblins are small, sneaky, and lazy.  Bugbears are even more sneaky and lazy, and a whole lot harder to keep disciplined, but they fight well.  Hobgoblins will have trouble keeping their troops in order.

Their wars will be no good for anyone or anything around them.  Good and Evil become largely irrelevant.  Nobody outside the wars wants either side to win.

It could be amusing if the ones secretly behind it all are the Humans.  Humans are commonly Neutral, the dominant race, the most civilized, and most powerful.  How better to deal with their enemies than to get them to fight each-other?  It thins out their ranks, keeps them out of mischief, and nobody is going to miss the Orcs or the Goblinoids if they all kill each-other.

I have a similar theme, and I find aberrations are good representatives of chaos.

For now, I’ve been trying to think of monsters/enemies that are particularly linked to either Law or Chaos; thus far, I’ve only been able to think of a few, such as devils and demons.

Why not link some yourself? I imagine wolves with their pack hierarchy and coordinated hunts would fall on the law side. Eagles, as spirits of freedom and independence might choose chaos. While things such as animals are typically unaligned, war has a way of forcing us to a side.

It could be interesting to see where everyone falls along that divide, and it would really drive home the scope of the conflict.

Interesting thoughts.  I especially like the aberrations, modrons, etc., because the paladin PC won’t be able to directly identify their influence.  I completely forgot about his Divine Sense ability, so what was meant to be some light foreshadowing turned into a quest— they’re already hot on the trail of the cult of Mammon.  I certainly intend to use fiends and fey, and possibly celestials, but for now, I’m leaning towards things that he can’t identify right off the bat.

Keep the ideas coming!