Wars in the 31st Millennium are fought in all manner of unbearable conditions. Possibly the worst of which are those battles in space, on battlefields with little to no atmosphere or gravity, where event the most minor of attacks can spell certain doom. Whether on the hull of a space craft, through a breach in its hull, on an orbital platform or a small asteroid base void battles are by far the most dangerous theaters a battle can take place in.
Void War is a minor set of adjustments to regular games fought in the Secession to help mix games up a bit and to provide an extra dimension into the game. The core rules should always be used for Void War games, with players agreeing to use any Additional rules they like.
All weapons in a void war have the Rending special rule. Any weapon that already has the Rending special rule will instead Rend on a 5+. Models with hardened armour or a 2+ save are unaffected by this rule.
All blast weapons also gain the Pinning special rule. Any weapon that already has the pinning special rule gives a -1 to the pinning check.
All Difficult terrain will also count as Dangerous terrain as well.
Any cover saves taken that result in a '1' cause an extra D3 AP- auto wounds. Representing the added danger of shrapnel in space.
Any model without a helmet has its Sv reduced by 1.
All models may run an extra D6" (so roll 2d6 and add them together), if any dice on a run comes up as a '1' then all the models in the squad must make an armour save, vehicles take a glancing hit.
Open to the Void
Have areas of the table be Impassible terrain to reflect that there is no ground there. Skimmers, Flyers, Jetbikes, Jump troops and Jetpack troops may traverse it as normal (no dangerous terrain test needed).
Passages of Death
Due to how little of a table Space Hulk corridors cover treat all of them as open ground, with the gaps being Impassible (ignore penalties for Jump, Jetpack, Jetbikes or Skimmers) and all LoS is unblocked. However each room or corridor section that a shooting attack, or witchfire psychic power, passes through gives +2 to a models cover save.
Extermination also has some nifty Void Battle rules for Zone Mortalis games.