Developed long ago before even the Imperium existed, these troops were developed in parallel with the Skitarii Legiones of Mars. The Secutarii act as the honour guard and protectors of the Titan Legions. While the role of a Titan is to shatter a fortress or blast enemy formations to ruin, it is the role of the Secutarii to eradicate such threats its survivors might still pose as the Titan presses on towards important targets, whilst also guarding the crew of each Titan and preventing its enemies from getting inside their Titan. The Secutarii devote themselves to hunting down and slaying those who seek to flee from the Machine God's wrath. Also, they can magnetize their hands and boots in order to climb up and down the outside of their Titans, ensuring they don’t get left behind.
All Secutarii have unique forcefield generators built into their bodies like a bionic implant, resembling a backpack. The generator creates an atmospheric impedance barrier which is capable of deflecting projectiles and at greater density, even deflect heavy weapons fire. The secret to this device is that a single generator is usually of little effect, but when woven together with those around it their power amplifies. This also justifies operating like a literal phalanx.
Secutarii are split into three distinct groups.
Secutarii Hoplites are a type of Skitarii warrior of the Secutarii Titan Guard, equivalent of most anti-armor infantry found throughout the Galaxy. Their role is to mop up survivors left in the wake of the advance of the Titans of the Adeptus Mechanicus with short-range weaponry that nonetheless can devastate enemy armored vehicles.
These warriors typically go into battle with an Arc Lance and Mag-inverter Shield. Akin the arc mauls of the Skitarii Clade officers, and the far larger of the mighty Cerastus Knight-Lancers, the Arc Lance inflicts damage through short range blasts of coruscating energy that can burn through xeno technology and render mute the Machine Spirit of even the most heavily armored vehicle. Mag-inverter shields provide the copious power needed by Arc Lances, and resists blows which would otherwise slay its bearer instantly through utilizing a powerful and sophisticated inversion generator at the shield’s core.
Of course, why get close to a vehicle instead of using literally any anti-tank gun or missile? Because then they wouldn’t have an excuse for forming a phalanx and wasting a bunch of guys on one vehicle instead of just shooting it with a meltagun.
Crunchwise in 9th Edition, they are budget Skitarii with budget TH/SS. These guys melt to massed low strength firepower but can hold their own against a charge from a knight gallant. They are very strong in melee especially when buffed with the omniscient mask. They are also very points efficient for what they do, so don't underestimate the shooting of a large blob of arc lances. At S6 AP-1 per shot, or melee S+3 (S6) AP-1 hits that both do 1 damage each or changes to D3 vs vehicles, hoplities are the perfect unit to tie up vehicles and even infantry in melee combat. Anything short of Terminators will fear a charge from Hoplites.
Secutarii Peltast are a type of Skitarii warrior of the Secutarii Titan Guard, equivalent to most base-line infantry utilized by almost everyone. Their role is the same as the Hoplites which is to mop up any stragglers left after the devastation made by the Titans of the Adeptus Mechanicus. Peltasts are equipped with longer-ranged weaponry meant to counter enemy infantry. They seek targets of flesh and bone that would attempt harm on the great God-Machines of the Legio Titanicus.
These warriors typically go into battle with Galvanic-casters. The Secutarii Peltasts are equipped for all manner of targets. This electro-galvanically driven, multi-function projectile launcher has multiple firing modes that may fire rapid bursts of low velocity, razor-edged metallic flechettes; attack concealed targets with retina-burning ignis charges; fire an armor-piercing inert slug-shot at supersonic velocities; or even fire a barrage of blind canister shot to confuse and disorientate the enemy.
In 9th Edition, these are essentially Ad-mech Dark Reapers. 5++ makes them slightly sturdier than Vanguard. Their Galvanic Casters have 2 fire modes: An Assault 3 18" Assault 3, s4 for blending GEQs and a MEQ killing 24" Heavy 1, s5 ap2. Once per game they can give themselves -1 to hit instead of shooting. Their price makes them an arguably better alternative to the Skitarii Vanguards. Unlike other Skitarii the Alpha carries the special equipment and can't take both an EDT and a melee weapon.
Secutarii Axiarchs were a type of battlefield commander used by the Mechanicum's Secutarii guard. These officers displayed perfect dedication and application to the Secutarii's art of war, but also have served on a battlefield where a fallen Titan was successfully recovered or protected from the enemy. This is the most holy task a Secutarii can perform, and as such the Axiarch's are seen as blessed by the Omnissiah.
It should be noted that they look almost exactly the same as the Peltast, makes sense since these guys acted more like squad leaders and sergeants than they do, a completely different branch. For a while, no one knew what they looked like until the release of the Horus Heresy: Legions card game. Some were understandably underwhelmed that the Axiarch looked exactly the same as the Peltast.
Once this criteria has been met, an Axiarch is remade and upgraded into a being that is inhumanely stalwart and nigh-indestructible. They were typically equipped with an Arc Maul, Radium Pistol as well as Refractor Field and Titanshard Armour.
Rules for these guys only appeared in the Horus Heresy tabletop, here, they are Secutarii HQ with stats more befitting a Space Marine. Of note, beside the rules most Secutarii get are the Refractor Field, Radium Pistol, Arc Maul, and Titanshard Armor, which grants them not only their pimping statline and a 3+ Save, but also IWND. Axiarchs can also grant all Secutarii within the detachment one of four new rules: a 5+ FNP, Move through Cover, +1 to pen vehicles, or PE Infantry. These guys are far cheaper than even the Magos, but they're also a lot more specific in their use and in a force with few Secutarii they're rather wasted.