From Warhammer 40k - Lexicanum
Phase-iron is a rare metal alloy, produced during the Dark Age of Technology. It was used to bind psykers, searing their skin every time they tried to use their powers. By M41, the metal was exceptionally rare. One place it existed in abundance was inside the Null Keep on Neva, where Deacon Viktor LaHayn was conducting secret experiments on rogue psykers.
Describing his captivity in the Keep, Torris Vaun remarked, "imagine acid boring into you every time you tried to eat, breathe or speak."
- 1: Faith & Fire (Novel), Chapter Thirteen, pg. 276