A void exists beyond what we would know to be existance. However it has a direct affect on existance, it is a sort of cosmic glue, holding together rapidly spinnning galaxies, and controlling the rate at which the universe expands. Due to the infinite endlessness, a void plays many roles yet very few would you see, or even comprehend.
-How the Valheru mainipulate said force-
The Valheru avatar creates a sort of internal force. One equal to that of the nuclear fussion process of a star two, or three times that of -our- sun. Creating within a balance, as the voidal essence moves all within, and without a Valheru, with its infinite gravitational pull, as well as many other affects. Take away the void, and the Valheru explodes. Take away the internal force, and the Valheru implodes into the void itself (End Attack justification).
A Void hold no ties to any plane of existance, for it supercedes them. Anything that enters the Void, would find their tethers to all planes, severed. Yet, due to the fact they all hold a single bond to the planes it remains intact. Yet nothing that they can do would ever find its way 'out' of a Void.