The Dark Mechanicus is the Chaos-aligned counterpart to the Adeptus Mechanicus. While they were once ordinary Tech-Priests and Enginseers, for one reason or another, they now serve as the principle arms suppliers for the forces of Chaos. While some members may have joined by being corrupted like any other would-be follower of chaos, as had the original Fabricator-General of Mars during the Horus Heresy, many others were first exiled from the Adeptus Mechanicus for committing Tech-Heresy, and find working for the Dark Mechanicus more liberating.
Instead of worshiping the Machine-God as the Adeptus Mechanicus understands it, they instead identify their god in Chaos Undivided. They readily use warp-based weaponry and frequently create daemon engines by binding daemons to machinery. They also have entire legions of traitor Titans and Knights at their disposal.
The Dark Mechanicus is not well fleshed out in official lore apart from a few characters and tabletop units, and haven't spent much time outside of the Eye of Terror, so they aren't covered extensively in TTS either. Like their loyalist counterparts, they are overtly sexual about their love for technology, but they take it in a far more extreme direction by making crotch-guns.