Krios Battle Tank
The Krios Battle Tank is the main battle tank of the Adeptus Mechanicus, and unlike most Imperial Tank designs it is horrendously exposed, what with the cogs and gears being placed everywhere for the eye to see. You would think the Tech Priests would have the dignity to armor up their main battle tanks so that it could survive more than a few minutes in a war. Seriously who the hell designed this! Well, it's not like we haven't seen laughably insufficient tank suspension or ridiculously exposed treads on a war machine of the imperium before...although the Rhino/Predator looks just like a real life US military vehicle so maybe not that unrealistic. And technically the Leman Russ like almost all Imperial vehicles was not a tank STC design but was shoehorned into the role. The Krios has no such excuse.
Ranting aside, the Krios is an ancient and arcane tank that is durable and protected by interlocking energy fields (Which I believe off-sets its horribly exposed mechanics, although that is still not an excuse). Its most common armament is the Lightning Cannon which can fuck over most enemy vehicles and tanks, making it akin to a Space Marine Predator, except more incompetently designed.
On the other hand, its closer to a SPG (self-propelled gun) than a tank in design: its just a gun on treads, and its meant to go fast and flank rather than slug it out. Its also mentioned that its got a pretty big and powerful reactor for its size, and that it uses this to both keep its super-charged engines running and its cannon firing. Presumably its open design is for cooling, since the combination of hot engine and hot gun will melt it into scrap, causing the tending tech priest to have an aneurysm (or BSOD).
The main weapon for the Krios is the aforementioned Lightning Cannon, a laser-guided electromagnetic beam designed to focus on a target and fucking melt it. It has a pretty short range, though, so the lack of a turret means that it takes some work to compensate for that. But, you don't need to worry about ammo and compared to a las-cannon, it has a much higher rate of fire.
Yes, believe it or not, the Krios has another brother called the Krios Venator which changed the role of the Krios from an assault gun to a dedicated tank destroyer, swapping its Lightning Cannon (which already fucks vehicles) with a Pulsar-Fusil (Think a multi-chambered particle beam cannon designed to fire volleys of destructive energy bolts which detonate explosively upon hitting their target), like most Super OP weaponry, the Pulsar-Fusil is guarded by the Mechanicus, only using when they are dragged into a conflict.
The Krios Venator is comparable in function and tactical role to the Predator main battle tank that is used by other Imperial forces. Although the Krios lacks the more flexible traversable turret for its main weapon, the Krios makes up for this disadvantage with its greater speed and manoeuvrability.
On tabletop for Horus Heresy, the Pulsar-Fusil from the Krios Venator is a S7 AP3 large blast marine eating lightning gun onto a A9 AP2 ordnance 4 tank killer machine, all for just 25 extra points. They also have Autosimulacra and access to Anbaric claws and Volkite sentinels. If you felt the need to rush your tanks in and make them more deadly. With Mauler Bolt Cannons, Irradiation engines, the 3-shot battlecannon on the paladin, and more anti-MEQ, the Krios Venator is just a much better choice than their normal cousins. They, like their normal cousins have the same armored profile, which is a 13/12/10 3HP tank with a flare shield to boot.