Grey Knights, Inquisitors & Assassins - Collectors Guide
Grey Knights, Inquisitors and Assassins first appeared in the Rogue Trader period. They were not a separate range as such but seemed to appear loosely grouped as part of the "Forces of the Imperium".
Around 2003, Games Workshop released the Daemonhunters range, loosely based around a core of Grey Knights, and the Witchhunters range, loosely based around a core of Sisters of Battle. Both these ranges included Inquisitors and Assassins.
By 2006-7, the ranges had been regrouped with the bulk of the Inquisitors and other figures from the Daemonhumters and Witchhunters ranges then being grouped as part of the Grey Knights.