I have an idea for a campaign where a particular villain makes a deal with a an archduke of one of hell's layer and that archduke supplies/assists them with evil constructs or helps them build them/supplies the resources (was thinking of Dispater and Dis for this) are there any layers associated with evil constructs?
The Second Hell: Dis and its Iron City
The second layer of Baator is Dis, named for Dispater, its ruler since the beginning.¹ It is an endless, arid, barren wasteland around a single megacity, also named Dis—because it dominates the layer, to the point that many will tell you it is the entire layer. The Iron City might be the only place in Hell that is even remotely welcoming to travelers, and one of the greatest trading hubs in the Multiverse.²
Dis’s epithet is very literal. All streets, buildings, and amenities in the Iron City are made of iron—and that iron is all red-hot. (Fire resistance is necessary for all visitors.) It is constantly remodeled by devils and their slaves to curry favor with Dispater, and it churns out metalwork of all kinds at all times. The city is near-literally a forge itself, and contains within it an endless array of forges.
On the other hand, Dis does not have any strong association with constructs. Hell doesn’t tend to care for them—when devils want endless labor, they get the souls of the damned to do it. An unfeeling construct that never tires of its post and never bemoans its lot is a wasted opportunity in a place where suffering is power. The Iron City probably manufactures a fairly-large number of iron³ golems, just because it makes a lot of everything iron,³ but it isn’t something the City is known for in particular.
That said, particularly for a scheme in which actual devils are undesirable, manufacturing constructs does seem very in keeping with Dis. Mechanus, the plane of Law, manufactures inevitables, robots made from alloys of pure law, to enforce certain laws—Hell could do something similar.⁴ They are, after a fashion, just as lawful as Mechanus.
Or just corrupt some actual inevitables. If anyplace can do it, it’s Dis—the other thing that Dis manufactures is corruption, in the prison of Mentiri, where those who break Hell’s laws are tortured until they really embrace Hell’s evil work. A secret cell buried in the depths of Mentiri is the perfect place to work on corrupting the supposedly-incorruptible.
The best sources for information on the Iron City of Dis are 3rd edition supplements, Manual of the Planes⁵ and Fiendish Codex II: Tyrants of the Nine Hells. The 2nd edition supplement Guide to Hell is also useful. I don’t believe anything in 5e has covered Dis in any significant detail, but the Nine Hells are still nine, so Dis still exists.
Of the baatezu occupation of Hell, that is. Which will be “the beginning” as far as you’re concerned, at least while in Hell, if you know what’s good for ya. Bringing up any dark about a time before that is asking for trouble.
Comparable to the City of Brass, which between them are tied for second to Sigil, the City of Doors.
And other metals, and non-metals too, for that matter—Dis manufactures a lot of everything. Iron is just something of a specialty.
The exact process of manufacturing inevitables is a closely-guarded secret, so Hell wouldn’t be able to easily replicate it exactly, but the general idea, perhaps somewhat less refined, should be well within their capability.
Note that D&D has used the title “Manual of the Planes” several times for separate publications in different editions. So far as I know, the 3e one is the only one with significant information about Dis.
Not Mechanus, but...
The Clockwork Nirvana is not a great answer to this question, because while modrons can be corrupted, when they are, they no longer fit into the great machine that is Mechanus. Mechanus doesn’t have a place or the time for evil—and it doesn’t accept things that aren’t in their proper time and place. There’s a reason why Acererak’s pet pentadrone was in his tomb: it didn’t really have anywhere else to go.
Even corrupting some chunk of Mechanus itself is a poor answer, because:
That’s never happened, and that’s despite the Primus being killed and temporarily replaced by a being of supreme Evil and Chaos (Tenebrous, the god-slaying god that the Demon Prince Orcus became when he had the Last Word).
Even if it somehow did, that corrupted chunk would cease to be a part of Mechanus. Mechanus is Lawful Neutral—whatever is not, is not Mechanus. Pieces of planes can and do shift from one plane to another when things get too far out of whack—a third of Arcadia, the layer of Menausus, slid off and became part of Mechanus because the Harmonium, which headquartered there, was so adamantly non-good. If a cog of Mechanus becomes evil, it would also drop out of Mechanus.
Which itself might not be a bad idea. It wouldn’t—likely, or at least at first—go to Hell, because Acheron is in between, and frankly Baator wouldn’t have a place for it anyway—the Nine Hells will remain Nine. But Acheron could take it—one weird cog floating among the cubes is plausible. You might even miss it; Mechanus has a lot of cogs, and while the modrons would definitely notice, they wouldn’t go advertising it. And no can ever really be confident about what is or isn’t to be found in Acheron, being the way it is.
I still like Dis better, though. The Mechanus-cog-corrupted-into-Acheron approach is for if you really want to make up something wholesale; Dis is for if you want to fit this into existing lore.
If Mechanus is too much machine for evil, and Baator is too evil for machines, the plane in between them might seem plausible. And certainly, such a plane could well fit in the lawful-by-lawful-evil slot in the Great Wheel. But that plane isn’t Acheron.
Acheron is, instead, an endless, pointless, battlefield. There are no goals, nothing to protect, nothing to win, and no victories to be had. Souls there fight for nothing more than their own survival—not surviving for anything, but just because they aren’t willing to give up. (Until they do.) In many ways, Acheron is a mockery of chaos: there isn’t any particular order to the plane. There is no rhyme or reason to who lives or who dies. It may, in that sense, seem very chaotic. But all that randomness comes out in the wash, because none of it means anything. Nothing ever changes in Acheron because all these random events have no effect whatsoever on any grander destiny. The whole point of the plane is that without some kind of order giving things purpose, even absolute chaos becomes uniform, sterile, and ordered. (Chaos has its own mockery of law in the form of Carceri.)
So Acheron isn’t really associated with forges or constructs. It’s only by importing a corrupted Mechanus cog that you might introduce some of those elements. That, or a divine realm.
Mechanus, the Clockwork Nirvana.
There is no layer of Hell that really fits what you’re looking for. However, the outer plane of Mechanus seems to be right on theme; quoting from chapter 2 of the DMG (p. 66):
On Mechanus, law is reflected in a realm of clockwork gears, all interlocked and turning according to their measure. The cogs seem to be engaged in a calculation so vast that no deity can fathom its purpose. Mechanus embodies absolute order, and its influence can be felt on those who spend time here.
Modrons are the primary inhabitants of Mechanus. The plane is also home to the creator of the modrons: a godlike being called Primus.
While it is not an evil plane, rather, Lawful Neutral, its inhabitants are not incorruptible. In Tomb of Annihilation (p. 177), we learn that the archlich Acererak used modrons from Mechanus to bring Mechanus chains into the tomb and drive his machines:
To drive his infernal machinery, Acererak contaminated a workforce of modrons from the plane of Mechanus. Under the stewardship of a corrupt pentadrone, the modrons rerouted one of their great chains into the tomb. The sections of the shaft above and below the balconies exist outside of normal space, preventing the shaft from intersecting with the tomb’s other levels.
So while Mechanus does not generally feature evil constructs, it has the constructs you need, and they can be corrupted.
While Dis is probably the best answer, weapons and armor for the Blood War are forged in Avernus. The forge, which also serves as Bel's fortress, is carved out of a volcano.
Source: Descent into Avernus, Ch. 3 (p. 113)