A Null Field Matrix is a form of advanced Necron technology used to project a large anti-psychic field around itself, similar to that employed on a smaller scale by Gloom Prisms and the Nightshroud barriers created by Monolith phalanxes and Gauss Pylon networks.
Most commonly used to shield Necron Tomb Worlds from psychic disturbance, they display the Necron's mastery over technology and their ability to create a technological defence for nearly any threat they encounter.
As creatures long without any kind of spiritual essence, the Necrons cannot project their minds into the Warp or harness its power to any degree. They are unable to use the Warp to journey across the galaxy and are thoroughly bereft of any native psykers. In many ways, this is a boon, for the Empyrean is always a fickle servant, given to wreaking havoc on those who seek to claim its power.
Yet this absence of psychic ability enforces its own limitations, particularly when combating creatures to whom sorcery is as much a part of war as conventional munitions, as there is no surer defence against a psyker than another, more powerful, psyker.
To compensate, many Necron Tomb Worlds are shielded from psychic disturbance by vast Null Field Matrices. Developed during the War in Heaven, these networks of anti-psychic field emitters generate an unknown energy field that destabilises a psyker's connection to the Warp, rendering him unable to utilise his full power. Similarly, daemons in the presence of Null Field Matrices have a tendency to flicker in and out of existence, as if unable to maintain a solid foothold in reality.
In the later years of the 41st Millennium, the Null Field Matrix has also proven to have a deleterious effect upon the Tyranids. The vassals of the Hive Mind are not immune to the unsettling soulessness of the Necrons, and the Null Field Matrix only serves to exacerbate this effect on the normally inviolate Hive Mind. Alas for the Necrons, such defences are far from complete.
A Null Field Matrix requires incredible amounts of power to function properly and is a fragile device that must be hidden away at a Tomb World's very heart to prevent its destruction. Nonetheless, they stand as testaments to the Necrons' ability to create a technological defence against any form of attack, no matter its genesis.
Additionally, it was a further extension of this same technology that led to the Necrons' uncompleted Great Work, the creation of the network of anti-psychic pylons erected on Cadia and many other worlds across the galaxy that were intended to cut off access to the Warp by all inhabitants of realspace forever.
Luckily for the psykers of the galaxy, the Necrons entered the Great Sleep before this project could be completed, though it is possible that the Phaerons of the more active dynasties might like to see it completed...
- Codex: Necrons (5th Edition), pg. 16