Tech-Priest Manipulus is a member of the Tech-Priest Auxilia, of the Adeptus Mechanicus. Usually a senior member of the mechanicus' cult, the Manipulus is charged with different tasks, though they usually all revolve around combat and warfare. Most Manipulus are known as members of special operations teams. While they have access to very advanced gear and weaponry, they usually excell at close and short ranged combat, in contrast to other tech-priests, like the Dominus.
Also it has a very derpy model. Very, very derpy. Fortunately there's conversions for this.
On the Fluff
The first Manipulus was first introduced as Balphamus Vaulk, the leader of Theta-7, a kill team dedicated to finish problems as part of different imperial campaigns. Though it's well known that Balphamus Vaulk has his own plans and goals, his services are greatly appreciated considering the range of missions they can pull off, from assassination, to sabotage, to escort, to stealing materials and technologies... In particular his team has a focus on stealth, which he gets by hiring sicarian infiltrators or ruststalkers (one is stealthy because you don't see it, other is stealthy because it can see you).
On the Tabletop
Manipulus were first revealed for the Kill Team Commanders expansion. More information for KT can be found here.
I haven't tried to use it for 40k, someone else please fill this.
Not to worry friend, your local AdMech player is here to help! //Begin Canticle of Examination::The Omnissiah watches//
Ok so right off the bat, any claims based on this guy being "between a Dominus and Enginseer" only go so far as stats. He shares the same 6" movement, S4/T4, the 4W of the Enginseer, the 3A of the Dominus, the 8L of both, and the Dominus' 2+/5++ and self-healing. Also the healing ability for friendly <Forge World> and Knights, but that was obvious. Oh, and BS3+ and WS3+. So with a pretty meh statline, he's basically a slightly cheaper Dominus (at only 90 points, compared to 92 (wow, amazing)), with slightly better melee capabilities, insofar as his staff is S6, but only AP-1, but he also gets to pretend to be a miniature Belisarius Cawl and use his mechadendrites (S4, 1D), but only for d6 additional attacks.
But here's the thing: you're not bringing this guy for his melee. Sure it's ok, but its nothing to write home about. You're bringing this guy for his Galvanic Field, the buff he puts on his buddies, which has one of two effects that change until your next Movement Phase. You want snipers that can reach across the board? We got you. How about Neutron Lasers that can reach 54"? Done and done. This guy is a gunline's best friend! Oh, and if you decide that you can't be bothered with buffing your guns' ranges, he's also one of the only amendments for the AdMech's footslogging crutch, but only slightly (and until we get that sweet new transport!).
And his gun's are pretty ok too.
We'll start with the Galvanic Field; at the start of your movement phase, you pick one of two buffs, Bolster Warriors and Bolster Weapons, which last until your next movement phase, and then you get to pick again. Bolster Warriors is the movement option, and lets you add +1" to the movement of friendly <Forge World> units that start their movement within 6" of him. Also, units get to add 1 to Advance and Charge rolls when they are still within the aura (RAW, they don't get that advance and charge buff if they move out of the aura during movement). This one is pretty good for early board movement, helping to get your Skitarii up the board faster, or helping your Sicarians make that charge roll they desperately need to make.
As for Bolster Weapons, this one is arguably the better of the two, but its got its downsides. First and foremost, if you chose this one, the Manipulus is forced to stand still until your next movement phase, no moving whatsoever. Not so bad when he's buffing a gunline, but otherwise, avoid it unless you know he'll survive until he can get into cover. Anyway, what it does is that you get to add 6" to the range of any friendly <Forge World> units within 6" of him, but only if said guns already have a range of 24" or higher, otherwise its 3" for everything else. The possibilities of this are pretty great, cause it also affects Rapid Fire distance, and isn't limited to solely Infantry either. Slap this fucker next to some Protector Protocol Kastelans, and laugh as they ignore cover at 42". Or put him next to the aforementioned Neutron Lasers or Icarus Arrays, and giggle with glee as that Dakkajet gets shot down from across the board!
On to his guns, he's got one wargear choice: swap out his basic magnarail lance (Heavy 1, S7, AP-3, Dd3, 18"), for a transonic cannon (Assault d6, S4, AP-1, D2, 8", auto-hit). The lance has an ability where if the manipulus stands still for a turn (which he will, most of the time), you get to change the damage of the gun to a flat out 3, a nifty little elite killer, if I may say so myself. Alternatively, his cannon is a pretty good MEQ flamer, allowing you to get those damn Primaris kids off your Forge World's lawn a little easier. The best part is, however, is that these guns also get buffed by Bolster Weapons! An 11" flamer? Yes, please! At the end of the day, however, you're gonna pick the one for where you plan on keeping him for most of the game: leading the charge, or staying in the back.
Final verdict: a decent enough HQ, if a little pricy for his stats and what he brings. If you plan on playing your AdMech as gunline, or want to live out your fantasy as a Technosorcerer, this guy isn't a bad pick.