Why there were pylons on Cadia before the birth of Slaanesh?

Question

From the description of the "Eye of Terror" we know, that this is a relatively new phenomenon:

The Eye of Terror is not a naturally-occurring astronomical phenomenon. The Eye was created by the psychic shock wave that accompanied the birth of the fourth Chaos God Slaanesh, the Prince of Pleasure, during the Fall of the Aeldari over 11,000 Terran years ago.

At the same time, we know that the pylons are much, much older:

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.

We know, that the Pylons were quite efficient in keeping the Eye at bay and almost closed the Eye of Terror during the 13th Black crusade. But why have they been put there in the first place millions of years before? Was there (before the Birth of Slaanesh) at this region already some sort of natural vulnerability?

Answer 1

Cadia is not the only planet with pylons such as this, in fact the eye of terror has been surrounded by planets containing such pylons, Abaddon led several crusades out to destroy pylons on other planets before the 13th destroyed Cadia hoping to allow the eye of terror to spread.

Before the Eye of Terror was created this area of space was the center of the Eldar species. The Eldar where created by the Old Ones to help in the war against the Necrons therefor it can be assumed that this (eye of terror) region was historically a key part of the Old Ones Empire and the pylons are placed on "Border Worlds" between the Necrons and Old Ones. Possible an early warning system, or some sort of Dampener designed to try and negate the Old Ones Power. You assume the pylons where placed here to prevent the spreading f the eye of terror but that might just be a happy coincidence.

The events may have been. Necrons build pylons as a weapon/defense in the war with the Old Ones. Old Ones Die the remaining races are to young, to immature to understand what the pylons are. They also don't have the ability to dismantle them if they want. Eldar empire rises in the ashes of the Old Ones, I imagine they spread out and possibly even inhabited the worlds that contained Pylons. Fall of the Eldar Empire, Birth of Slaanesh, the eye of terror is created within the Eldar Region, the Pylons, as a side effect of there original purpose, act as a dampener to the Warp containing the eye of Terror and preventing it from spreading beyond, what was originally the Necron Border this was never there design or purpose but a happy coincidence based on the history of the region.

It may be the Pylon worlds throughout the galaxy denote the old Necron Empires border with the Old Ones? It just so happens that in this place they also happen to map to a major area of Warp Space.