Noctilith

Noctilith is the High Gothic term coined by the Adeptus Mechanicus for Blackstone. Yes, that Blackstone, the same stuff that was used to construct the self-same Blackstone Fortresses from Battlefleet Gothic, as well as the mysterious Pylons found on Cadia.

Despite being native to the Material realm, Blackstone interacts naturally with the energies of the Warp. As the experiments of Belisarius Cawl soon discovered, the material reacts differently depending on the polarity used; it either repels the warp, as it did on Cadia, or magnifies Warp energy exponentially, creating the Warp Beams that the Fortresses are so infamous for.

In the wake of the creation of the Great Rift, there's a mad scramble among magos to try and find more of the stuff. Much to their surprise, they've found caches of Blackstone on many of their Forge Worlds.

The problem? Where there's Blackstone, there's sure to be Necrons, and as many Magos found to their horror, many Forge Worlds were built on Necron Tomb Worlds. And if the likes of Tyranids give something a wide birth for the threat they pose (as they do more and more with the Necrons of late), then that should tell you something.

Obviously, the Necrons don't want to share, since most of their Anti-Warp tech require Blackstone to work; it doesn't help that the Crypteks that originally built the Pylons and other tech are only just beginning to awaken (and even then most of them have damaged memories so it's unclear if they can still remember how to make new ones). Not surprisingly, open war broke out, but it is a war that the Mechanicus is slowly losing, as more and more Tomb Worlds stir...

And, per the HH novel “Mortis” - Big E has his own supply, tucked away in the bowels of the Palace, in the form of a prison where he and Malcador keep some especially strange things/people safe and secure.

The Corrupted Shards

Shards of the Cadian Pylons were retrieved from the ruins of Cadia, and after corrupted by profane rituals, used to seal off the main Warp Routes out of the Sol system. If not for a rogue operation by Adeptus Custodes and Sisters of Silence, this would have isolated Roboute Guilliman on Terra before he could launch the Indomitus Crusade.

The Noctilith Crown

First unveiled on Vigilus, the Noctilith Crown is a gate created out of Blackstone used to weaken the borders between worlds, which allow Daemons easier purchase in the material world. As one of Abaddon's bag of tricks, it's obviously used to devastating effect on the Imperial defenders there.