The Skitarii are the cyborg armies of the Adeptus Mechanicus, constitued of augmented cybernetic warriors.

Ingame, Skitarii are often mentioned as holding the line against the Necron tides so that your Tech-Priest cohort can enter the various tombs to explore. Eventually, you can recruit a variety of Skitarii to join the cohort, as playable units. At the beginning of the game, only the Servitor is available, while others must be randomly unlocked and upgraded through missions. Which means that sadly I cannot provide an unlock guide, only a description guide.

As the Skitarii commander, Khepra very often rewards you with units in her quests.

Skitarii require cognition points (CP) to deploy in battle, and blackstone to be recruited for a mission (excepted the Servitor). Should they survive, you will earn back a portion of the blackstone fee depending on their remaining HP. This fee is the only downside to them dying, so they are much more expendable than your Tech-Priests. Besides, mission rewards can further lower the blackstone cost.

The "upgraded" Skitarii (level 2 and level 3) are unlocked like any other troop, as a mission reward. They keep the same costs but gain abilities or stat buffs.

They provide significant help in most missions, so they should not be overlooked. Especially with the right support. A Tech-Priest with the "Secutor" branch can significantly buff Skitarii using its array of support skills. In particular, the Command Fire ability gives one free attack to all troops in range, focused on a single target. The Reinforcements ability allows to summon cheaply a single unit, for example a melee troop directly next to a Necron Deathmark, or one of your own snipers for quick eliminations. The summoned troop will often (but not always) move right after your Tech-Priest.

• Role: Meatshield
  
• Combat: Melee

The Servitor is a mindless cyborg that was programmed for very simple task.

You have Servitors from the very beginning of the game. They serve as meatshield: they are cheap, and you gain CP every time they suffer damage. A Servitor getting hit twice was already 100% profitable.

Servitors can lock-down an enemy ranged unit in melee, and get an additional Opportunity attack when it tries to move away. Pretty useful in the early game against Necron Warriors.

At Level 2, the Servitor gains +1 HP and +1 Damage, plus a "Taunt" ability attracting enemy attacks, gaining +1 physical/energy armour for one round.

At Level 3, the Servitor gains +3 HP and +2 Damage.

• Role: DPS
  
• Combat: Ranged

The Vanguard is a basic ranged unit, and has one of the few Physical ranged attacks. Its damage output can be surprisingly high, especially at higher levels.

In my experience, the Vanguard benefits enormously from buffs from Tech-Priests, allowing it to dash through the map and deal very high damage from afar. Vulnerable in melee.

At Level 2, it gains +1 HP and an active ability allowing it to shoot twice in a single turn, allowing for high burst damage.

At Level 3, it gains +1 HP, +1 physical and energy armour, +2 damage, +3m movement and +10% critical chance.

• Role: DPS, Crowd Control
  
• Combat: Melee

The Vanguard Alpha is one of few Skitarii with AoE damage, allowing it to take on several enemies at once such as swarms of Flayed Ones. Although the residual damage doesn't quite allow it to score multi-kills, it should be considered as softening the other targets.

As with other melee units, it can lock down ranged Necrons and score opportunity attacks against them.

At Level 2, it gains +1 HP, +1 physical and energy armour, and an ability to immobilise a target. If used on a ranged Necron, it will be unable to attack for a full turn if the Vanguard Alpha stays on it.

At Level 3, it gains +1 HP, +2 Damage, +10% critical and +3m movement.

• Role: Sniper, Support
  
• Combat: Ranged

The Ranger is a sniper unit, simply put. It deals average damage but has very long range, and reveals enemy statistics when hitting them.

Thanks to its range, it can soften or finish enemies across the map without fear for retaliation, and can support from afar your more mobile units.

Very useful against Necron Deathmarks.

At Level 2, it gains +1 HP, +1 energy armour, and an ability to disable an enemy weapon for one round. This ability also has long range and reveals statistics, and is valuable support.

At Level 3, it gains +1 HP, +1 physical armour, and most importantly a 100% critical hit rate. Meaning its shot will always deal 4 DMG, ignore armour and prevent a falling Necron to raise again.

• Role: DPS
  
• Combat: Melee

The Ranger Alpha is a melee unit with high damage potential. As with other melee units, it can lock down ranged Necrons and score opportunity attacks when they try to move away.

Initially, it has the ability to evade opportunity attacks, allowing it to run through Necron lines and attack important targets in the back.

Its machine spirit attack is charged after only one use and breaks enemy armour.

At Level 2, it gains +1 HP, +1 Damage, +1 Physical Armour. It also gains the ability (1 CP) to dodge 100% of attacks from a targeted enemy, for one round, allowing it to take more dangerous necrons 1-on-1.

At Level 3, it gains +1 HP, +1 Energy Armour, and the ability to strike twice per round (without cooldown), giving it high damage potential.

• Role: Nuker
  
• Combat: Melee

The Kataphron is one of the two end-game Skitarii units, being unlocked rather late and with a higher CP and blackstone cost than the others. Having a mechanised chassis, it is a large unit taking 4 tiles instead of 1 (cannot take narrow paths)

The Kataphron is straightforward: its only attack deals AoE damage in a large circle around the target. The residual damage being the same as the direct one, it can put a strong dent in a large group of enemies. Especially given that the damage is high. Be cautious however, the AoE can strike your own units.

It is recommended to summon the Kataphron using the "Reinforcement" ability (rank 9 Secutor), so it will take only 1 CP instead of 3 and be placed where it is the most needed. It benefits enormously from the Secutor boosting abilities, e.g. a level 3 cognitive canister to give it a 100% critical hit AoE attack with strong damage.

At Level 2, it gains +2 Physical Armour

At Level 3, it gains extra damage on its machine spirit attack (which takes however 2 turns to charge).

• Role: Nuker, Tank
  
• Combat: Mixed

The Kastelan Robot is a precious relic from the Dark Age of Technology.

It is the second of the end-game Skitarii, being unlocked pretty late and having a high cost (4 CP to summon). It also occupies 4 tiles on a map and cannot follow narrow paths.

Its main attack is a flamethrower with a wide cone-like AoE and heavy damage. It can soften or kill a large group of enemies, which will also suffer from Burn for a few turns. A perfect tool for purifying xenos.
In case of need, it has a melee attack although given the unit size, the melee damage is a bit disappointing.

The robot doubles down as a tank due to its HP pool and its special ability to gain +2 physical/energy armour for one round.

Summoning it with the Secutor "Reinforcements" ability allows an optimal placement and reduces significantly the cost (1 CP instead of 5). Further buffing it with e.g. cognitive canisters leads to hilarious results.

At Level 2, it gains a directional charge allowing to pin down enemy units.

At /Level 3, it gains +5 HP, +2 physical armour, +2 energy armour, and an additional flame-thrower attack (second-to-last in the screenshot)

• Role: DPS, Berzeker
  
• Combat: Melee

A melee unit with the unique ability of ignoring armour when striking, making it extremely useful against heavy targets.

The Ruststalker becomes stronger with time, as each attack increases its critical hit chances and (at level 3) its total damage. Attacks on downed necrons count for the bonus.

At Level 2, it gains +1 HP, +1 energy armour, and the passive ability to evade Opportunity Attacks

At Level 3, it gains +1 HP, +1 physical armour, and +1 damage per attack done in the current battle

• Role: Scout?
  
• Combat: Ranged

The Infiltrator starts the battle in the "Untargetable" state: it cannot be attacked, neither in melee (opportunity attacks) nor ranged, although it can still be hit by AoE attacks. This allows the Infiltrator to move freely through the map and always score the first hit.

Its damage output is however comparatively low. Best used against weaker units or e.g. against a Necron Deathmark.

At Level 2, it gains +2 HP and the ability to become untargetable again. Allowing it to attack, then switch back to invulnerability. The ability has a 3 turns cooldown however.

At Level 3, it gains +2 HP, +1 physical armour