A Cadian Pylon is one of several thousand massive pylon-shaped devices of unknown xenos origin erected millions of standard years ago in a complex network stretching across the surface of the Fortress World of Cadia. These pylons can also be found on a number of other worlds close to the great Eye of Terror Warp rift in the Segmentum Obscurus.
In a galaxy replete with mysteries, the Cadian Pylons are amongst the most enduring. There are over five thousand such edifices scattered across the surface of Cadia, each one standing some five hundred Terran yards above the surface, and reaching two hundred and fifty yards below. Reports differ, but it is understood that there could be anywhere between two and three thousand more concealed below ground as the result of tectonic movement down the ages.
Despite millennia of study, the Adeptus Mechanicus long failed to discover the purpose of the pylons. Servitors sent within invariably ceased to function or suffered circuit overload; all attempts to breach the structures' gleaming surfaces met with failure. Any recovered data was fragmentary at best, and contradictory at worst. Even the identity of the pylons' creators was shrouded in mystery.
Some amongst the Cult Mechanicus believed the spires to be the work of the undying Necrons, or their mortal antecedents the Necrontyr, but then there were those on Mars equally convinced that the pylons were constructed by the Old Ones for the sole purpose of destroying the Necrons and their former C'tan overlords. The one thing all investigators agreed upon is that the pylons were responsible for the stable Warp-corridor known as the Cadian Gate. Adepts conjectured that they emitted a becalming signal, taming the roiling energies of the Immaterium.
In the wake of the events of the 13th Black Crusade and the destruction of Cadia, it is now known by the savants of the Imperium of Man and the Eldar Craftworlds that the pylons were erected over 65 million solar years ago by the Necrons during their ancient war with the Old Ones to serve as a defence against the psychic abilities of the Warp -- the Necrons' only real vulnerability. The pylons essentially acted as anti-psychic field emitters, restricting the activity of the Immaterium in proximity to them.
The mysterious pylons of Cadia have stood since long before the coming of Man to this remote world. But their ultimate purpose remains a mystery. It is currently believed by the savants of the Imperium of Man and the Eldar Craftworlds that the pylons were erected over 65 million standard years ago by the Necrons during their ancient war with the Old Ones to serve as a defence against the psychic abilities of the Warp which are the Necrons' only real vulnerability. The pylons essentially act as anti-psychic field emitters, restricting the activity of the Immaterium in proximity to them. The Tech-priests of the Imperium's Adeptus Mechanicus have hypothesised that the pylons on Cadia exert some effect over the Immaterium, rendering it calm and creating the unusual zone of space-temporal stability known as the Cadian Gate, that enables ships to travel through the only known navigable route in and out of the Eye of Terror into realspace.
Recently, it has become clear to Imperial savants that the pylons can be reduced to ruin by overloading their capability to hold the Empyrean at bay with massive amounts of Warp-spawned enemies, such as a massive incursion of daemons into realspace at one time. When the first signs of the coming Black Crusade began, as Warp Storm Baphomael expanded and began to engulf the edges of the Cadian System, of the 5,180 known intact pylons, at least 1,292 began to emit a psycho-temporal resonance. During the 13th Black Crusade in 999.M41, Abaddon the Despoiler intended to overload the pylon network on Cadia itself to allow the Eye of Terror to envelop the worlds of the Imperium, making it possible for that Warp Storm to encompass the entirety of the galaxy. This outcome is known to the Forces of Chaos as the Crimson Path, for without the Cadian Pylons even Terra itself ran the risk of being swept up in a rapidly expanding Eye of Terror.
And this is exactly what happened at the climax of the 13th Black Crusade. As the Archmagos Dominus Belisarius Cawl, with the aid of the Necron Lord Trazyn the Infinite, successfully activated the Cadian Pylon network's full anti-psychic capabilities to begin to actually close the Eye of Terror, Abaddon the Despoiler used the damaged bulk of the Blackstone Fortress Will of Eternity as an artificial asteroid, unleashing a kinetic strike on Cadia. The force of the massive star fortress' impact with Cadia destroyed the fields of pylons across the planet, disrupting Cawl's attempt. The sudden failure of the pylon network allowed the Eye of Terror to begin to expand across the galaxy, swallowing up the mortally wounded world of Cadia even as tectonic stresses ripped it apart. The rapidly expansion of the Eye of Terror provided an anchor point for the eventual development of the Great Rift that cut the Imperium in half after the end of the 13th Great Crusade.
The Cadian Pylons are massive structures of smooth black stone, 500 metres high and 250 metres square. The upper facing of each pylon contains holes and other round orifices about the size of a human head. The wind moves through these holes and often produces an eerie, mournful sound. These tubes through the structures are not straight. Mechanicus Tech-priests investigating the pylons have attempted over the centuries to run Servitor probes through them to map the structures but usually they did not return. There are 5,810 fully intact pylons on the surface of Cadia and over 2,000 others that are partially ruined or buried relics. No two of the pylons are identical in design largely because the strange tubes that run through them each follow unique patterns that differ from pylon to pylon. Each one rises exactly 500 metres high and is sunk 250 metres below the planet's surface. The similar pylons on other worlds in the Cadian Sector near the Eye of Terror look identical to those found on Cadia's surface.
The Cadian Pylons tended to be clustered in large fields of many monoliths, including such regions of Cadia as the Elysion Fields, which possessed the largest concentration of the xenos constructs, as well as at the lesser sites of Kasarn, Trosk and Vorg.
- Codex: Chaos Space Marines (6th Edition), pp. 19-20
- Codex: Eye of Terror (3rd Edition)
- The 13th Black Crusade (Background Book), pp. 14-15
- Eisenhorn (Novel) by Dan Abnett
- The Gathering Storm - Part One - Fall of Cadia (7th Edition), pp. 10, 68-88