The Ordo Reductor is a highly specialized military division in the Mechanicum who focuses solely on destruction instead of creation or knowledge, and has existed ever since the Mechanicum was brought into the fold of the greater Imperium, when the Emperor went to Mars to sign the treaty with the Lords of the Red planet. Their purpose is to destroy all those who dared opposed the Great Crusade, exterminating xenos, observing and analyzing technology to obliterate it, and to penetrate impregnable fortresses. So the Iron Warriors aren't the masters of siege - the Ordo Reductor are. Yay.
Few in number, the Magos Reductor in charge of these divisions were often psychotic, violent madmen or dangerously on the side of heretical. But thanks to these traits, they excel at killing, which is why the Mechanicum usually don't regard them with suspicion and distrust as they do against the intelligent machines of the Legio Cybernetica. Damn.
Nomadic in nature, the Ordo Reductor are usually based on specialized warships, commanding Convenants that are made up of attendant servants and war machines. These Galleass class ships are self-sustaining, essentially a mobile forge world of its own, and were forged in the vast shipyards of Saturn. Apparently the Ark-Mechanicum class is more common, but anyway, the Galleass class ships function as transport, forge and permanent abse for the Ordo Reductor, being the Magos Reductor's sovereign domains that they guard zealously even from their fellow Mechanicum priests. Like this they go from battlefield to battlefield, fighting under diffeerent banners and commanders to destroy stuff.
They not only are warriors, but also military intelligence, their data, knowledge and tactics often put to good use by Space Marine Legions. So the Ordo Reductor are rarely seen to wage war directly aside from targeted raids, probing strikes or scouts, as well as tallying the dead and collecting stuff for their studies once the battle was over. Besides this, they, like I said earlier, are masters of siegecraft, being the foremost experts that surpass even the Iron Warriors and Imperial Fists, and many Primarchs. Their history is full of accounts where they conquered unconquerable defenses by sieging them into oblivion. Awesome, eh?
The Ordo Reductor were also given special permission to use forbidden weapons from the Dark Age of Technology and dangerous arcana, as well as to innovate new weapons and war machines to annihilate the foes of the Great Crusade. That's how they got the Land Raider Achilles, now known as the Macrocarid Explorator, and the Hellion-Minoris artillery for the Centurio Ordinatus (so they did existed back then!). They also created the Thallax cybernetic shock troops so that we don't need to use Skitarii models for Mechanicum armies and are forced to buy them from Forge World. Jokes aside, the Thallax were created because the Ordo Reductor had no access to the Skitarii Legions of Mars and other Forge Worlds, being isolated on a floating large ship in space. Poor guys, must be lonely out there, with no worlds to recruit their troops from. Well, apparently they traded away or gave their Thallax to other factions of the Mechanicum, that's why we can field them in a Taghmata army and Legio Cybernetica army, as well as the Myrmidon sub-cult, often for supply and allegiance. This then led to more power and more independence, but at the cost of Mars's direct military control over outer Forge Worlds.
Several Covenants, and even a handful of demi-Legio made of a few Convenants, ended up becoming loyal to certain Space Marine Legions, in fact incorporating their heraldry and livery into their stuff, and they're proud to serve beside these Smurfs...I mean Space Marines. So when the Horus Heresy split apart the Imperium, many of these Ordo Reductor Convenants sided with their adopted Legions in becoming Loyalists and Traitors. Fortunately, unlike the Titan Legions or Martian Mechanicum, where the majority sided with the Traitors, the Ordo Reductor seemed to remember the purpose of their creation and most of them backed Terra instead. Perhaps some of these dudes merely saw Warmaster Horus as the greatest challenge to their arts of war and so decided to fight him for fun, much like we Warhammer 40K players do today.