I've got two ideas, both for the Emperor's Children:
"Excruicator Terminatus" Sonic Cannon.
A parasitic daemon weapon that is implanted within the chassis of a vehicle, the Excruciator Terminus resembles a bizarre hybrid between a vox-amp and a siege gun, though most of its body is contained within the vehicle it infests. It fills the hull of its host with fleshy cables and metallic tentacles, corrupting every single component and giving the bound daemon within the cannon control over the machine.
The weapon itself is terrifying to behold, producing crushing waves of sound that causes every cell in a victim's body to simultaneously burst, usually reducing them to bloody slop. Armour is seldom a defence against it, for it can also produce a high-pitched shriek that shatters adamantium like glass. Worst of all, the host vehicle is heavily fortified by the daemon possessing it, rendering it nigh-on indestructible and able to regenerate from damage that would reduce any other vehicle to scrap.
Fortunately for the Imperium, such vehicles are rare, and not without disadvantages; they become useless as transports due to their interiors being filled in their entirety with the daemon-weapon's innards, and they require a "minder" at all times to prevent them from turning on their allies. Land Raiders are common hosts for these abominable weapons, though there have been records of a Typhon Siege Tank being converted.
Take any tank. Put a big sonic blaster of some kind on it and blend it on with cables (guitar wire, GSW tentacle makers, etc). For bonus points, add extra cable/speakery details to the vehicle.
Use an artillery tank of some kind (Vindicators for example) and you've basically got yourself the basis. I actually got the idea from converting up my Land Raider into a "Noise Raider", which will be proxied as an Achilles.
Pillar of Debasement:
Fluff: Appearing as an abstract floating column, the Pillar of Debasement takes many forms depending on its origins, and no two Pillars are the same. Some appear as primitive idols, barely recognizable as machines at all, whilst others resemble intricate crystalline rods carved with arcane geometric circuitry and overlaid with shifting murals of incredible artifice, and everything in between. However, at their core, all Pillars of Debasement are creations of monstrous evil with the singular goal of the corruption of all things to the path of excess, debauchery and ultimately ruin.
The Pillar of Debasement projects a powerful psychic field around it which gnaws at the souls of mortals- any sentient being who spends too long near the Pillar will have their mind assailed with whispered voices promising unearthly delights and visions of exquisite pleasures of all kinds. Quickly, social mores are abandoned, vows are forgotten and the weak willed are in the thrall of the Pillar. They will do its every whim, turning on their foes or taking their own lives if the foul device so desires. Even those with a strong mental fortitude may fall before its mental onslaught, for resisting it will bring about its wrath, filling the offending mind with visions so horrific that most will never recover from the trauma, if indeed they survive the experience.
It is unknown how far the influence of the Pillar extends, for even artillery commanders ordered to fire on the things have found themselves under its sway. However, whilst its corrupting power is terrifying, the machine is physically mostly defenceless and is fairly frail- those few who can see through its illusions can destroy it fairly easily.
Modelling: Self explanatory really. It's a tall floaty thing that doesn't have a set look. So basically, short of putting it on a flying stand, go wild.
Rules: None at present. Not sure how you'd even write rules for this thing really...