Previously only available through the June 2010 white dwarf issue, GW has now posted these as a free download. Here is the link.
The Night Spinner is unique amongst artillery vehicles. It does not fire crude explosive shells like the guns of lesser races, but instead utilises a far more subtle and elegant means of delivering death from afar. The Night Spinner grav-tank mounts a doomweaver atop its sleek chassis, a weapon that spins threads of monofilament wire into a lethal web that is launched high into the air. This substance is incredibly thin, and yet so strong that nothing can stop its inexorable descent towards its victims. Indeed, the threads are known amongst the Eldar as the Chain of Vaul, named after the unbreakable bonds of legend.