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So, what would be the best way to go about representing the Dark Mechanicum on the tabletop?

Forgeworld mechanicum force led by archmagos draykavac. That's the main dark mechanicum in the fluff. You can only do dark mech with that HQ and he's a beast. Give him an abeyant and 4 bodyguards, get him into combat and watch him wreck a primarch. I recommend support from a thanatar (should probably be your paragon), castellax with darkfire cannons and thallax with photon thrusters as your compulsory troops. Fill in points with krios venators and more thallax (4 squads of 3x each is good). Myrmidons with your choice of loadout in a Triaros is also good. If you want a flyer, the lightning with Kraken missiles is the only one worth taking but 30k Ad Mech lacks scoring already so I only recommend it in large games. If you have trouble with heavy armor, knight atrapos is awesome.

What models do you have/want. The FW route is most accurate (but expensive) but still fairly early heresy options. You can do a good amount of crazy gribbly mech nonsense, with the CSM, daemons and renegades codexs.

If you want "proper" 40k darkmech there are two paths:

1-count as with regular admech, but that sucks.

2-using CSM "Purge" detachment. now your mandatory is HQ and two elites, and you can pick more heavy than normal (can still take troops if you want, but no fast attack)
Being unable to take any mark other than nurgle is not a problem for this army.

Then, for HQ you take a warpsmith, your (mandatory) elites can be helbrutes/decimators/etc you got 6 slots of these.
And then, the heavy section you got 4 slots, and TONS of options.
You can also add some cultists if you feel like it, but without obsec, they probably not worth much. and a lond of war, that section has plenty of darkmech constructs. (or chaos knight?)

As long you stick to non-marine choices, it will look like a darkmech army, and even some marine picks will make sense (plague marines, oblits and mutis can be modeled like some darkmech "servitors", etc.)

If you're going the FW mechanicum direction you can also use 'Inmar Satarael' for dark mechanicum - he/it sided with horus.

I had heard a while back that FW regard the mechanicum list for 30k to be dark mechanicum in 40k.

When it comes to Dark Mechanicum, I think there are a variety of routes. First you need to decide the era you want it to represent. Heresy era "Dark Mechanicum" aren't as crazy as 40k era "Dark Mechanicum".

Heresy era Dark Mechanicum, at least at the point in the Heresy FW has gotten, aren't that different from normal Mechanicum. Its more just a philosophical difference with a greater willingness to use extreme surgery on tech thralls and their soldiers and greater use of "disposable" units like Automata.

It seems in both eras, from their representation in fiction there is less "middle management" among the dark mechanicum; you have an even more extremely oligarchical leadership directly enforcing its will over the masses.

In 40k... you'd probably want to mix as many daemon engines from the CSM with the Skitarii and Dark Mehcanicum; using Warsmiths and Oblitrators to represent your Dark Mechanicum techy-priesthood. You could mix in FW's Chaos Renegades, using their rules for a Heretek leader and the bionic enhanced slave masses.

In 30k, you would probably want an army predominantly of Automata, Techthralls, Myrmadons, with an Archmagos. You could then bring in through the Imperial Militia "cybernetic augmented" combined with one of the other Provenances of War to represent either their Skitarii equivalents or masses of slaves, or the subjects of their experiments.

New and varied Deamon engines, that goes to Medium automata suits animated by dead corpses of of Dead Champions to Large hulking Mech Monstrosities of Brass, flesh and Doom.

Supported by units of Thralls that works best when they are in drove " proximity network data sharing" or something.

With some open topped transports on tracks or legs( don't care), and all of it Battle bros with CSM/Deamons/KDK of course( so that we can have cheap ass open topped transports...)