Saint Gerstahl was an Imperial Army officer who had fallen defending the Cadian Gate during the first centuries after the Horus Heresy. Entombed on the Shrine World named for him -- Gerstahl -- under the world's alabaster capital, the saint was prophesied to rise again when the "Eye of Darkness opened for the last time" and stem the tide of the treacherous.
During the 3rd Black Crusade, Abaddon the Despoiler, the Warmaster of Chaos, launched an attack on Cadia, unleashing the Daemon Prince Tallomin, the so-called "Prince of Princes," against the Cadian Gate, in order to distract the Imperium of Man from the desecration of Gerstahl's tomb. Abaddon destroyed the remains of Saint Gerstahl and forestalled any chance of the prophecy's fulfillment and his return as a Living Saint.
- Black Legion - A Codex: Chaos Space Marines Supplement (6th Edition) (Digital Edition), pg. 32