Thanks for the mock up Hendrik :) It's funny, that was kind of how I was imagining it really. I plan to use the Mechanicus head like in that picture but shave it down similarly to how Clone did it in his ETL thread (had that idea for a long time and his work gave me a visual of how cool it looked). Two sets of horns, one where you put it and another set like tusks coming from where the mask cuts in at the jaw. Looking at Forge Worlds brass etch at the moment, think it could replace the green stuffing you suggested, or if not, can add some awesome details. Then plan to cover the knight in spikes and chains and a few other Chaos-y bits before painting it like my Scions/Veterans and Word Bearers and covering it in painted script like my tanks.
As for the base, I am either thinking of using one of the Quake Cannon Craters (if it will fit), I'll have his foot in the middle and some dead stuff so it looks like he just stomped everything or use the big, broken aquila from that Imperium Terrain kit with the knights leg bent at the knee, stepping on it. Unsure which to go for at the moment though.
How does all that sound though? Hopefully that is enough of a description to give you a good picture of it.
Thanks Guys, I think I am going to have to get one, they are so cool and from all this feedback, it sounds like they will do great too. I almost knee-jerked into it myself until I saw that it could not ally with chaos which is my main army, and talk of a future Daemon Knight release made me hesitant. But now anyone can take it, it seems unlikely that will be released now so I am going to go for it! ;D
*I'll definitely have to find that tutorial then as I wanted mine bent but thought it was part of the kit to do that as so many people had.