Most video games these days that are set in the Warhammer: 40,000 universe focus on the most well-known element in that setting: Space Marines. This is not one of those. The player takes on the role of an Imperial Inquisitor: an agent of the Imperium of Man tasked with hunting down and destroying heretics, aliens, mutants, unsanctioned psykers and traitors. Inquisitors have nigh-unlimited authority and power, and are supported by various Inquisitorial orders of the Imperium.
Source: N4G PC Warhammer 40,000: Inquisitor – Martyr review – ChristCenteredGamer