"YOU MUST CONSTRUCT ADDITIONAL PYLONS."
- – Protoss executor, 2500, for the hundredth time, to a player who doesn’t realize he needs to build more fucking pylons
Necron Pylons are Sentry Pylon's older brother. Like its smaller brother, the Necron Pylon look a lot like a crescent shaped Necron "defense turret" constructed from the same living metal as the Necron Monolith. And like the Sentry Pylon, the Necron Pylon is not a traditional turret in a sense as the Pylon's position is not fixed and Necron attacks are often supported by Pylons that are teleported to the surface of the besieged world. When used in a defensive manner, Pylons emerge from the ground to attack trespassers and subsequently vanish to escape retaliation. Pylons have a bloody large range of fire thanks to their ability to rapidly traverse the firing element of their primary weapon system, making them a threat to both land-based targets and aircraft alike.
In terms of armament, the Necron Pylon has a power matrix similar to that of a Monolith, from which the Pylon draws energy prior to discharging it in one of two forms: as a Particle Accelerator attack, or a Gauss Flux Arc. A Particle Accelerator is essentially a larger version of the Monolith's Particle Whip, and is the result of the Pylon focusing the energy from the power matrix into a single bolt rather than distributing the energy on the Monolith's four cannons. On the other hand the Gauss Flux Arc is more diffuse, coruscating discharge of a lower strength than that of the Particle Accelerator, and is thus capable of affecting multiple units at once so think of them as a giant Gauss shotgun/flamer.
In the fluff, these things are the Necrons' designated ship-killers, meant for taking chunks out of Strike Cruisers and the like.
Another variant of the Necron Pylon is the Gauss Pylon. They only differ in weaponry however, as the Gauss Pylon has a Gauss Annihilator, which is capable of firing a Focussed Beam (similar to the Particle Accelerator attack, however stronger) or projecting a Gauss Flux Arc. The Gauss Annihilator is also capable of rapid traverse, and when working in conjunction with the Gauss Pylon's impressive target lock and guidance systems; is able to shoot down almost anything, Super-Heavies included.
Also installed in each Gauss Pylon is a Phase Shift Generator. Which is another Necron Holo-field. A Phase Shift Generator allows nearby Necrons to occasionally shrug off fire that would usually render their armour useless as protection.