A Cadian Pylon is one of a large number of massive pylon-shaped devices of unknown xenos origin erected millions of standard years ago in a complex network stretching across the surface of Cadia and a number of other worlds close to the great Eye of Terror Warp rift in the Segmentum Obscurus. 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 it is the activity of the pylons on Cadia and the other worlds near the Eye of Terror that are responsible for creating the unusual zone of space-time stability known as the Cadian Gate that is the only known navigable route 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 enemy, such as by a massive onslaught of daemons into realspace at one time. In the wake of the 13th Black Crusade in 999.M41, Abaddon the Despoiler intends to overload the Pylon network on Cadia itself to allow the Eye of Terror to envelop the worlds of the Imperium, allowing the 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 runs the risk of being swept up in the Eye of Terror.
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 these probes usually 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.
- Codex: Chaos Space Marines (6th Edition), pp. 19-20
- Codex: Eye of Terror (3rd Edition)
- The 13th Black Crusade (Background Book), pg. 15
- Eisenhorn (Novel) by Dan Abnett