Just got my hands on a copy of the Tome of Decay (I also ran a Black Crusade game) and I got an interesting idea for a major event in my campaign involving a Daemon engine. Essentially a chaos warband using the shell of an ancient Ordinatus as the body and the players having to wreck it...now I have some rough ideas as to what I want to do with it and all that, but I am playing with some plans for Psy rating for it, for practical reasons allowing it to go beyond 9 is probably best but I want to get other opinions on this, as well as thoughts others might have.

Unless you tell us those plans, I'm not sure anyone will be able to really weigh in.

I have a hard time envisioning why anyone would need to have a basic Psy Rating above 9 or 10. If you feel it appropriate for your campaign, there is no absolute reason why you couldn't do it, though.

As the GM, it is your prerogative to discard and apply rules as you see fit. If it makes sense for your campaign, go for it. I can see an Ordinatus being turned into a Daemon Engine, possessed by a Greater Daemon of Tzeentch, manned by an army of cultists and sorcerers, covered by Screamers of Tzeentch clenching against the outer hull.

Boosted by cult sacrifice and myriad sorcerers within the now-derelict and twisted command bridge, the Ordinatus Daemon Engine fires enormous, devastating bolts of warpflame and twisting energy against the mundane warslaves of the corpse-emperor.

And while it could be entirely appropriate at this point to have an obscene Psy Rating, and Pushing the powers like mad, you might as well just homebrew it's powers, narrating what happens. Let's say that thing is pushing out 15d10 with an effective Focus Power of, what? 155?

Does it really matter anymore to the narrative if you roll or not, or wouldn't it be better to describe how it devastates the landscape, unless you're going to have the players roll up to it in a tank or something? Nevermind that I doubt it'd hit individual targets.

But it largely depends on what you want to do, and I have no idea what that is.

Fair enough, I guess my fear is my players reading this, but I suppose if I want help I need to risk it. Basic idea is that there is a world constantly wracked by storms, the actual reason has to do with malfunctioning archeotech in the cities beneath the earths blasted surface, the cities aren't really entered, crawling with monsters and large collections of mutants.

The Daemon engine Ordinatus is being used by a Warband, the thing is not just a massive platform of death with a Changer of the Ways bound within, for the Ordinatus can take control of the great storms on the planet as well, and worse yet it can harness them for a massive ritual that will mimic the one in the Black Crusade Hand of Corruption adventure that sends a world hurtling through the warp. Aside from bringing the world to the eye of terror it would also be able to bring down a massive EMP storm on any forces on that world, frying electronics, tech priests, etc. The boosted psy rating would be around 14-15 and also provides an actual bonus to any utilizing the warp, followers of tzeench get the bonus with no (or at least fewer) problems, those that do not follow also have a much greater chance of generating perils of the warp.

The current thing is I'm not sure, one idea might be that the players might discover it while on a mission into one of the undercities, chasing a mutant horde that raided an Imperium post on planet. While down there they may also discover caches of archeotech weapons but discover the warband creating this thing, at that point they are likely going to have to either get to the surface ASAP warn the commanders and pray, or they will have to try to keep it from the surface with bombs, guns and whatever they can scrounge.

I agree that the damage should be narrative damage but a psy rating that high outdoes the Great Unclean One I believe, so do you want it to be that mentally powerful? Also then you don't have to roll in order to see that mountain fortress crushed by a psychic bolt, it just happens in the "cutscene"