Just what are the Demiurg? They've been part of Warhammer 40,000 canon since the release of Battlefleet Gothic back in 1999. Some have seen them as an attempt to re-introduce the Space Dwarf archetype after the unfortunate fate which befell the Squats. Others have noted the similarities between the Demiurg and the Bentusi race from the Homeworld series (NB, save Homeworld!).
Anyway, while there are rumours about the Demiurg making an appearance when the Tau codex is revised, you can play as Demiurg in Battlefleet Gothic. There are two ship types that are available from Forgeworld: the massive Stronghold ship, and the smaller Bastion cruiser.
Several years ago I bought four of these smaller ships with the intention of giving them to a friend as a christmas present. Shamefully, they've languished in the queue since then. But now I've finally finished them!
The paint job is very simple: Army Painter Uniform grey, washed with a 50/50 mix of Badab Black and water, then drybrushed with Coat d'Arms Slate Grey, and finally Citadel Rakarth Flesh. These models are a dry-brusher's wet dream. I picked out some of the surface detail using Tin Bitz or Gunmetal, which I then washed with Devlan Mud.