Grail Thralls are grotesque, formerly human soldiers of the Cult of the Black Grail, one of the three main Heretic factions of Trench Crusade. Victims of the Black Grail plague, Grail Thralls are what happens when a victim doesn't have the strength or will to withstand the Black Grail's effects, their bodies turning into monstrous, bloated walking corpses, the skin tearing to reveal diseased flesh beneath. These husks are used by the Black Grail nobility as disposable shock troopers.
There are two variants of Grail Thrall. The traditional Grail Thrall has a massively increased pain tolerance, being close to impervious for pain. The second is known as a Fly Thrall, and sacrifices their pain tolerance, though they still have a higher pain threshold than regular humans, for for greater mobility. This is due to the massive hell-flies, which burrow their proboscises into the Thrall's central nervous system, giving the fly control over their victim.