The dark mechanicum, dark mechanicus, or the new mechanicum as they refer to themselves, are the adeptus mechanicus that turned to chaos. It is where mixing the biological and technological is such good heresy.
Sometimes rumoured that they will have their own 40k codex, they are a great excuse to mix and match inquisitor bits together (especially the chaos henchmen accessory pack that was officially released for inquisitor).
With pin vice and clippers in hand, i started work on some Hereteks (i.e. renegade tech-priests) and their traitorous creations such as Death-masques (think chaos spawn meets combat servitor) and Redemption Servitors (aka heretek arcoflagellants).
I also have some more death masques that need painting up (using drukhari grosteque bits as a base) . And if i can stop faffing about, I plan to make a Negavolt Cultist and some Biomass Servitors (using FW Mioptic Spores).