Pylons disrupt the Hivemind

• 5th Ed Codex [Necron]:

"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."