A Grey Knights Prognosticar is a powerful, highly specialised Astartes psyker who serves within the Grey Knights Space Marine Chapter and who are particularly sensitive to fluctuations in the Immaterium. They read psychic tremors to predict the location and severity of forthcoming daemonic incursions. Based on the information they are able to gather from tremors within the Warp, they can predict a daemonic incursion before it happens.
At the height of the tallest tower of the Grey Knights' fortress-monastery, high enough that its spire pierces the toxic skies of Titan, sits the aerie, known as the Augurium, of the Grey Knights known as Prognosticars. Few souls, even amongst the Grey Knights, ever set foot in the fortress-moastery’s Augurium. Sixteen thousand steps lead from the base of the tower to its pinnacle, which can also be reached by a slow lift ride, or in cases of emergency, a teleportation platform. Once a Grey Knight arrives at the top of the tower he emerges into a preparation chamber, the marble floor patterned in pink veins, with two Paladins arrayed in Terminator Armour standing watch before a great gate of blackened bronze. The marble with pink veins in the stone is said to come from Terra itself, for this kind of marble does not form naturally anywhere else in the Sol System. The bronze gateway opens in stages, rattling and clanking, accompanied by the withdrawal of sliding bars that keep the portal sealed shut. When it finally rumbles open at last, both doors grind back on heavy tracks, close to three metres wide. Within this sacred candle-lit chamber lies the inner sanctum sanctorum of the Prognosticators.
Within its mirrored walls, the Grey Knights' Prognosticars go about their rituals and meditations to the light of guttering candles. Prognosticars are powerful, highly specialised psykers, who are particularly sensitive to fluctuations in the Warp and who read psychic tremors in th Immaterium to predict the location and severity of forthcoming daemonic incursions. Only those Knight-Brothers that show the most promise with their powerful, latent psychic abilities are chosen for this onerous duty. To be pulled from service within a squad and isolated from the rest of the Knighthood is said to be an honour, of course. Such a position carries rank and responsibility, but it also comes with its share of sacrifice. No one wished to stand alone on the fringes of an order founded on the collective strength provided by brotherhood
Whilst the skills of the Prognosticars might seem oracular in form, in function they are much more akin to highly specialised hunting instincts, tracking the unholy spoor of daemons through the roiling tides of the Immaterium. It is a Prognosticar’s duty to hunt through the Sea of Souls, seeking the ripples caused by the Archenemy’s greatest intrusions. Forewarned by the Prognosticars, the Grey Knights can deploy their forces according to the nature of the threat at hand, rather than relying on delayed or misleading reports from a battle zone. Though such predictions are prone to a certain amount of inaccuracy, the Prognosticars provide the Chapter with a level of foresight without which its mission would be all but impossible.
If a Grey Knight commander orders the destruction of a passenger liner to ensure the banishment of a single daemon hidden aboard, he does not do so out of callousness, but out of pragmatism. Thanks to the warnings of the Prognosticars he knows that the escape of that daemon would lead to the damnation of far more souls than those that perished in the liner's destruction. In the Grey Knights' war against the daemons, the tally of the slain can only ever increase -- all the Sons of Titan can do is moderate the collateral damage. If millions must be sacrificed to save billions, then so be it.
- Codex: Grey Knights (5th Edition), pg. 9
- The Emperor's Gift (Novel) by Aaron Dembski-Bowden