Omnissiah for Dummies: How to take Mechanicum in Your Horus Heresy Army

Mechanicum are one of the many incredibly unique features that the Age of Darkness has to offer: the combination of industrial robots, technology driven body-horror, and crazy Victorian-stylized automotives make it easy to see why the faction is a draw to many people. While many players choose to go the good old fashioned route of building out a primary detachment with them, the Mechanicum are unique in that there are quite a few options you have for adding some Omnissiah-flavored goodness to your army list without having to build an entire army. We’ll cover all of the ways you can pack some bots in your lists, as well as pick out some notable units and army construction tools that can help support them. We won’t go too deep on any specific tactics or strategies: the goal is to cover all of the options you have for list construction with both flavorful and functional inclusion of our friends from the forge worlds.

First up, we’ll go through all of the ways you can actually include some of these units in your army construction. We’ll mention options for Legion Specific Automata units, but we won’t go into detail on including them; even though they’re functionally similar to units in the Mechanicum army roster, they are accessible just by playing that specific legion. After that, we’ll cover a few key things that don’t explicitly let you bring anything from the Mechanicum roster on their own, but are useful to know for synergies and powerful unit combinations.

Credit: Robert “TheChirurgeon” Jones

How can I take Mechanicum Units?

Allied Mechanicum Detachments

Starting off with the obvious, I know. It’s worth covering here due to how many factions can ally with Mechanicum: pretty much every faction has a minimum of Fellow Warriors with them. Iron Warriors, Iron Hands, Salamanders, Raven Guard, and Sons of Horus are Sworn Brothers, which opens up even more options for synergistic choices allowing for things like Battlesmith(X) and some Warlord Traits to carry over to the other faction.

One of the main benefits of taking an allied detachment of Mech is that you can access an Archmagos; while they aren’t particularly cheap, they allow you to have some more options with building around a particular type of unit in the Mechanicum roster through Orders of the High Techno-Arcana. If you want to read more about the specifics of these, we’ve covered them here in our review of the Liber Mechanicum. One major Order worth noting is Cybernetica, which is the primary way to give Line to Castellax maniples, although each one has a unique flavor to bring to the table.

+ Allows the most freedom in unit choices

+ Accessible to most armies

+ Up to 9 total Force Organization slots to utilize

– Requires at minimum of 1 HQ unit and 1 Troop to fill the detachment

– An Archmagos can be expensive if you want to access Orders of the High Techno-Arcana

Alpha Legion Praevian. Credit: Lupe

Legion Praevian

Praevians are an upgrade for a Legion Centurion that gives them a Cortex Controller and the Legiones Cybernetica rule, as well as the ability to take a Cyber Familiar as an additional upgrade. The special rule they gain allows them to take a single maniple of Castellax or Vorax Automata using the same force organization slot as themselves. Notably, this unit gains the Legiones Astartes (X) special rule for the legion of the Praevian, however they cannot take any Legion specific wargear. From The Depths of Tredecimmia Exemplary Battles PDF, Traitor legions now have the ability to instead take a Blood Slaughterer or Decimator instead of the Automata, but doing so uses a Fast Attack or Heavy Support slot respectively.

The Praevian is a really nice way to add a little bit of flavor to your list if you want a more niche role to be filled by the automata. While Vorax can be a bit clunky to use here since they will outpace the Praevian with his Cortex Controller to keep them in check, the other options can be great list additions. Castellax provide a solid weapons platform on an incredibly durable body, Blood Slaughterers are a great missile unit to use as a speedy threat, and Decimators rival Leviathan Dreadnoughts in bulk and brawn.

+ Almost always accessible to Legiones Astartes lists with a low entry fee

+ Legiones Astartes (X) on your maniple

+ Traitor legions can pack some unique units

– Takes up an HQ Slot

– Aside from the Cortex Controller is just a regular Centurion

– Fairly restrictive unit selection

Legion Forge Lord

Forge Lords are another Consul upgrade for a centurion, similar to the Praevian. They get a Machinator Array, and with it access to the Battlesmith rule, letting them attempt to repair vehicles and automata. They can access a Cortex Controller as well as a Cyber Familiar for additional points on top of this. For special rules, they have the Master of Automata rule, letting them join Automata units (unlike the Praevian), and when doing so gain Fearless and lose access to reactions, as well as giving them a pseudo Cortex Controller effect.

Forge Lords also allow you to take up to three units of Thallax as elites, with a similar clause to the Praevian in that the units gain Legiones Astartes (X) without access to legion wargear. It’s worth noting that Thallax are not Automata; they’re effectively cyborgs, but they’re so far on the side of Robot that it would make Robocop blush.

These guys are interesting in that they aren’t necessarily meant to be taken on their own, but provide a lot of benefit if you have other ways of putting Automata in your list to make best use of the Battlesmith + Cortex support. Thallax are also a sweet little speedy infantry unit that can be built to harass tanks and infantry alike; they keep their native Line, so it can be a good way to bolster your objective control with a mobile unit like this.

+ Battlesmith and Cortex are great tools to help your Mechanicum units

+ Thallax provide fast and dangerous scoring units, just keep them out of reach from scary melee units

– Takes up an HQ slot

– Doesn’t natively provide Automata, just Thallax – to make best use of one, you’ll need another way to bring some or have some in your existing army list

Rites of War: Brethren of Iron

Brethren of Iron is a rite that represents every legion’s ties to the Forge Worlds that supply them, and thus is available to any Legion naturally. BoI allows you to take Castellax, Vorax, Domitars, and a single Thanatar as non-compulsory units in their respective slots. Legion Techmarines can also shell out 20 points for a Cortex Controller to hang around with them. For limitations, they aren’t too restrictive:

  • You can’t include more automata units than non-automata units
  • A mandatory Forge Lord must be taken as an HQ choice. This means that this rite is locked out of Allied detachments, since you’ll also need something with Master of the Legion to gain access to the rite initially
  • You need a Cortex Controller for every 3 Automata models you include, meaning Techmarines, Praevians, and even further Forge Lords will be necessary depending on how many units you want of Automata.
  • No Paragon of Metal upgrades can be taken. Making your robotic build-a-bear primarch is a secret the tech priests plan to keep.

If you want to run Automata but want to do it in a more points efficient way than taking an allied detachment and don’t already have another Rite of War you want to build around, this is a solid choice. This also is a nice option if you already have an allied detachment but still want Automata support. This will give you the most options for different units without needing to take an entirely new detachment, and thus offers a nice amount of flexibility with a smaller amount of extra investment.

Another interesting choice is actually taking allies as well as BoI: this can open up selections such as a Thanatar (from BoI) as well as an additional Heavy Support option. For armies with Sworn Brother level allies with Mechanicum, this can be a pretty nice boon.

+ High flexibility in list construction

+ Low initial points investment for the amount of units you have access to

+ Relatively unrestrictive rite

– Mandatory HQ selection and Cortex Controller tax forces you to take additional units if you want a lot of Automata

– Takes up Rite of War slot, locking out Legion specific options

– Needs a bit more support to be as effective as solely bringing an allied detachment of Mechanicum

Credit: Robert “TheChirurgeon” Jones

Rites of War: Head of the Gorgon

While I don’t want to cover legion specific units that are natively in army lists, HotG is a Rite for the Iron Hands legion that allows you to give stubborn to infantry in your deployment zone, outflank any vehicles, and replace Flamers with Graviton Shredders. In addition, the Rite allows you to take Castellax maniples as elite choices and lets Iron Fathers (a Praetor Upgrade we’ll cover in a bit) and Techmarines take Cortex Controllers, notably at a slightly cheaper rate than Brethren of Iron.

The major difference between this and bringing a unit with, say, a Praevian, is that these specifically do not get Legiones Astartes (X) and instead gain It Will Not Die (5+). There tends to be a lot of debate over whether this is more useful than having the Iron Hands legion trait, which reduces the strength of incoming shooting attacks by 1. In truth, the answer is really just subjective, since it depends heavily on the type of army on the other side of the table. If the weapons they choose to bring and target the unit with aren’t impacted as much by the strength reduction (i.e, a Lascannon, being that Castellax are Toughness 6), then IWND will help keep the unit running if they can’t wipe them out in one go, but against an army packing a lot of S6 & S7 Fire, bringing the wound roll down will help mitigate a lot of the incoming damage before it even happens.

In conclusion, it’s a bit hard to determine the better option in all situations, and in reality you probably aren’t picking this Rite just to take the Castellax, but if you do want to take some and already are in this Rite, it’s a nice way to supply a few.

Credit: Robert “TheChirurgeon” Jones

Solar Auxilla: Iron Pattern Cohort

Moving away from the Legiones Astartes, we have the Iron Pattern Cohort option for Solar Auxilla, accessible through taking a Legate Marshall. This one’s pretty simple: you gain access to Castellax and Thallax as elites, your Marshalls and Auxilla Captains can take Cortex Controllers, and your Legate Marshall gains Master of Automata to roll around with a few Castellax bodyguards. The drawbacks are that you’re limited to one each of both Armored and Artillery tercios, so you’ll need to rely on some of these units to pack more heavy weapons.

This can be a nice way to gain access to a lot of cheap Cortex coverage, and while taking up elites isn’t the best, Solar Auxilla are much more efficient in regards to slots in the force organization chart than the Legions. This also is the only way to get a Solar Aux unit in with Automata, as the faction doesn’t have Sworn Brothers with Mechanicum.

+ Lots of Cortex Controllers

+ Great heavy weapon support for a faction that benefits from it on non-vehicle options

+ Really durable bodyguards for your Marshall

+ Provides actually decent mass infantry, something Mechanicum natively lacks alongside Automata

– Less vehicles overall

– Not as much general support for Automata in the roster

– Doesn’t buff any of the army’s existing units, making it compete with Cohort Doctrines that do

Imperialis Militia: Liberation of Constanix II Provenance – Forge Remnants

This is one of the newer and less known options, but Militia recently got an expansion to their Provenance list in the Liberation of Constanix II PDF, giving them some forge-world themed options to play around with. Of them, they have the Forge Remnants: a more elite mechanized infantry focused Provenance (having some nice synergy with Warrior Elite off the bat if you want to run just Grenadiers) that allows Grenadiers and Command squads of below 20 models to take Triaros Conveyers. This is a sweet deal, as Triaros are fantastic transport options that provide quite a lot of defense for a cheaper price tag than something like a Land Raider, which you could access through the Survivors of the Dark Age Provenance.

On top of this, you get three extra heavy support slots to toy with, but they have to be Krios tanks from the Mechanicum list. All of these gain the Third-Line type and go to BS3, so they’ll be a bit less potent than taking them naturally in Mech, but this is a great way to provide some solid armor to your Militia list without having to invest in more expensive options through other Provenances or bringing an allied detachment for more firepower.

Credit: Robert “TheChirurgeon” Jones

Legion Specific Automata

Some of the Astartes Legions have access to certain Automata just through their army roster. You usually don’t need to do anything specific to take them, but taking some units that can synergize well with Automata can help boost their effectiveness.

  • Thousand Sons – Castellax-Achea Automata: These bad boys are a solid option for Thousand Sons players, coming typically as an Elite, but available as a troop in The Achaean Configuration Rite of War. Normally, they’re a Castellax with a bit more firepower, Hammer of Wrath (1) and Adamantium Will(4+), as well as the ability to be considered the origin point for a nearby Psychic attack, letting you slingshot some spells off their location. Psychers can also count as a psuedo-Cortex Controller for them, which is a nice bonus. In The Achaean Configuration, they also gain Line when nearby other Psychers and can have wounds from nearby Perils of the Warp rolls get redirected to them. They’re a nice alternative to traditional Castellax that really shine in their dedicated rite, but can pull weight outside of that if you need some beef in your roster.
  • Iron Warriors – Iron Circle Maniple: This variant on the Domitar chassis is available as an Elites option or as a Retinue for Peterabo. Compared to normal Domitars, they have more durability from their shields and the Moving Bulwark ability letting them form a shield wall in base-to-base contact with each other for a 4++ invulnerable save. They have a Shrapnel cannon in place of the Missile Launcher, as well as the Graviton Maul hitting at S9 AP2 with Brutal(2) and Haywire, a seriously scary threat. They still suffer from the harsh reality of WS4 being crippling when you’re typically going to be fighting more elite units, so they are best served as a really durable frontline or bodyguard unit that can pummel vehicles and pin infantry squads. They can really bully other WS4 units, such as other Automata or non-legion Terminators with ease, but they do have quite the price tag, so be weary of sending them into any targets that might be able to take them down or abuse their slower movement due to Heavy.
  • Dark Angels – Excindio Battle Automata: These things are big, comparatively slow, and absolutely a walking nightmare if it gets close to anything. Basically an uncaged robotic beast, this behemoth has the unique Shackled-Artificia unit typing, making it basically an automata for all intents and purposes but also having access to Hatred(Everything) and Rage(2), but can never be scoring. It comes with an array of stupidly deadly weapons to deal with most types of targets and will eat anything it touches alive in melee, coming base with 6 (or 5 if you pick up a gun instead of the other big arm) thunder hammer-equivalent attacks at WS6, as well as a pair of combi-bolters that can be swapped out for even more firepower. The caviat to all this is the Vengeful Rage ability, that makes it lash out at whatever is nearby if it fails a leadership check after taking a wound, meaning you need to keep this away from your main forces. That being said, with a 2+ save, It Will Not Die(4+), and a 5+ Invulnurable save, it’ll be a slog to get through all 6 wounds. It isn’t cheap though; 350 points without any upgrades is a hefty cost to pay, and you’ll need to make sure it can have some help actually getting to combat since it isn’t particularly quick with only 8″ of movement and heavy. Vengeful Rage will sometimes make it creep a bit closer to opponents if you position it cleverly.

How Can I Support Mechanicum Units?

Now that we know how to get access to some Mechanicum toys to play around with, let’s highlight a few units and army construction tools that we haven’t already mentioned that naturally have good synergy with them. It’s worth mentioning that a lot of these will rely on being Sworn Brothers if you plan on using Mechanicum units in a different detachment than these to get the most benefit.

  • Cyber Augmetics Militia Provenance allows you to gain Sworn Brothers with Mechanicum, so if you’re playing Militia and want to use an allied detachment or vice-versa, this is a great pick. It also gives all your eligible models Slow and Purposeful as well as Feel No Pain (6+). Personally, I like to pair this with the Armory of Old Night Provenance; this will give you access to Lasrifles in place of your Lasguns, which won’t have any downsides due to being Heavy now! It gives you some other sweet weapon options as well, including Volkite and Needle weapons on Grenadiers and Command Squads.
  • Iron Fathers for Iron Hands detachments make a really good unit to hang around Mechnicum units, since they will provide a serious melee threat while being able to Battlesmith well and hoist around a Cortex Controller in the Head of the Gorgon RoW.
  • Questoris Knight Household Detachments or Knights taken as a Lord of War can benefit from any bonuses you’re able to share with them if you have Sworn Brothers, since they share the same allegiances as Mechanicum. It can be a nice way to give them access to Battlesmith support or other synergies that will effect vehicles.
  • Even if you’re not using a Rite of War that needs them, Forge Lords and Techmarines can be very beneficial to stick around your Automata to keep them in the fight. Keep in mind that not all units can join Automata, but you can hop over to them and repair them just fine.
  • Artillery & Armored Vehicles can be nice to benefit from things like Artificia Machina abilities and the preposterous amount of Battlesmith units Mechanicum has access to.
  • Melee & Screening Units for Castellax can be very helpful, as their lower WS and Initiative makes them struggle up close, even with decent weaponry. Since most of the suggested ways of bringing Mechanicum units tend to favor Castellax and other Automata, it’s worth keeping something on hand to help them stay out of the fight.

Credit: Robert “TheChirurgeon” Jones

There are tons of other fantastic synergies and options available from the Mechnaicum roster that are worth checking out, especially if you’re choosing to rock with an Allied detachment of them. We’ll cover them in more detail in the future with some specific loadouts and more detailed unit strategies, but if you want an idea of how to use some of the units check out our existing review of Liber Mechanicum here.

Have any questions or feedback? Drop us a note in the comments below or email us at