Warhammer 40K: Inquisitor - Martyr is expanding with Inquisitor - Prophecy, but as a standalone expansion. Inquisitor - Martyr will not be required to play, though "the story is designed to fit both for new and returning players". Inquisitor - Prophecy features the new Tech-Adept Inquisitor class, a brand new campaign that continues the story of Inquisitor - Martyr as players travel through new environment to face off against the Eldar and Tyranids.
Gameplay is also being reworked:
Inquisitor – Prophecy will be based on Inquisitor - Martyr’s 2.0 update, bringing the base game’s new level cap, redesigned loot-system and leveling, its new end-game, the co-op campaign mode, new items and a much faster, ARPG-focused gameplay to the stand-alone expansion as well.
Warhammer: 40K: Inquisitor - Prophecy will be released for PC on May 28th for $25 for new purchases. Those who own Inquisitor - Martyr will receive 30% off the price. The console version of the new expansion will be released at a later time.
Learn more on the Warhammer 40K: Inquisitor - Martyr site.