I'm running a Logician/Phaenonite/Tzeentchain campaign, and I'm a little lost on how to actually represent Malifica, that is, techno-arcana fuelled by warp energy. The ultimate goal is to have the party face down a warp-fuelled automata of some description, but I have no idea how to actually represent the fact that it is warp-fuelled rules-wise. Any ideas?

The easiest thing to do would probably be to give a warp-fueled automaton the Daemonic Trait, with all of the corresponding pros (Daemonic Presence) and cons (vulnerability to Holy weapons). To drive home the unnatural-ness, you could give it some distinctly un-mechanical Traits, such as Regeneration (maybe have the damaged metal grow back covered with spikes).

As for how to represent it to your players, I would describe it as looking like a brass 'steampunk' monstrocity, but instead of smoke stacks billowing sparks and black smoke, they are belching forth unholy purple flames and clouds of brimstone...

Don't stop on giving it only the "Daemonic" Trait, give it also the "Daemonic Presence" Trait. Flesh it further out with a few phenomena taking place in it's vicinity, like the various effects daemons have on reality (as seen in Daemon Hunter). Give it a few Psychic Powers (unless it's a construct of Khorne of course).

If you want it to be even more Nasty, Warp Weapons all the way (or something nasty you made up yourself, like a warp-spewer inflicting corruption instead of damage like the Flamer of Tzeentch).

Either way, I'd really recommend taking a look at Daemon Hunter, it has some nice ideas concerning Daemons, how they manifest and how they're cast back to the Abyss they're from.

These are some great ideas, thanks. I'll hold off on the psychic powers, but the regeneration thing sounds solid. Maybe something similar to the Slaugh regen, since a warp-fuelled automata would probably have a lot of wounds to regenerate. I think Fear could work as well, and I like the idea of Warp Weaponry, and I'll take the corruption-inflicting ranged weapon idea onboard

I shall definately have a look at Daemon Hunter, and I'll browse The Laythes to see if I can find any more inspiration

Also considering re-skinning some major mutations.

Corpulent could easily represent an excess of cables and pistons that have 'grown' from the exposure to warp and baleful energies. Hellspawn wraps up nicely as is, Necrophage could represent it needing to maintain it's 'fuel' source, deriving it's energy not from potentia coils but mortal flesh and souls.